Here is a little history and background on the Aluminum Block Slant Six engine.
The 170 and the 225 SL6 were both introduced for the 1960 model year. The 225 was originally installed in larger cars only, and the 170 was originally installed in just Valiants. Both engines were released as cast iron blocks only.
Midway through the 1961 model year, Valiants (and the new Dodge Lancer) could be ordered with the 225 as an option to the 170. These were still all cast iron engines. A few months later, the aluminum 225 was released as the predominant optional engine (a few iron 225s were also used) for Valiants and Lancers. It is likely that a few aluminum 225s were also installed in larger cars. There were a couple of engineering-prototype aluminum 170s, but none were installed in production cars. The prototype aluminum block 170s were made using the sand casting process. The aluminum Slant Six block was series-produced in the RG (raised or tall block) 225 configuration using a die cast process. This block configuration can be used with 225 crank/rods and with 198 crank/rods. Of course, the 198 was still years away (1970-74) when the aluminum engines were produced. I mention that the 198 crank and rods are for use in an RG block specifically because so many sources screw this up and call the 198 an LG engine (Low block, like a 170, with the lower block deck height). In summary, the aluminum 225 block will accept any 198 or 225 forged crankshaft (not the post-'76 cast cranks). Of course, the appropriate rods / pistons must be used with whatever crank is installed.
The aluminum 225 was installed in passenger cars from mid 1961 through early 1963 model years. These R, S and T series blocks (corresponding to 1961, 1962 and 1963 installation) will be found in the field. Early blocks have 12 bosses cast into the RH side oil galley of the block, in anticipation of the abandoned idea to use hydraulic lifters, being fed by oil directly from the main oil galley. Later Aluminum blocks (from mid '62) do not have these bosses. You can find what sequence number Aluminum SL6 you have by looking at the number stamped into the passenger side of the rear bell housing mounting flange. Approximately 45,000 Aluminum Block 225 engines were produced.
All of the aluminum 225s have casted-in iron bore liners and came with iron cylinder heads. Initially the heads had the word "SPECIAL" cast in where the casting number was usually placed (above number 5 intake/exhaust runners outside the valve cover rail). Later, a five-pointed star was used to denote the special head for use with the aluminum block, which had slightly smaller combustion chambers and tighter control of combustion chamber placement to assure proper head gasket sealing. This was necessary due to the relatively narrow area available at the top of each cylinder liner for gasket sealing. For the 1963 model year, the tighter placement control and smaller chamber diameter were communized on all Slant-6 heads; any '63 or later cylinder head can be used on an aluminum 225. It is very wise to install the head on the empty block during engine buildup and take the time to manipulate the locating pins and sockets to get combustion chamber placement perfectly centered over each cylinder. Aluminum blocks, in their stock form, can be over-bored a maximum of 0.040". Beyond this, special modification is required.
The aluminum SL6 engine uses a completely different head gasket, so a regular iron-engine head gasket will NOT work. These special head gaskets are no longer available through over-the-counter parts store channels, but any good gasket specialist (advertising in e.g. Hemmings) can supply the special gasket. It is also good to find and use the rear main seal auxiliary sealing parts specific to this engine. These can also be obtained from gasket specialty houses.
It is interesting to note that the placement of the connecting rod oil squirt hole in the Aluminum SL6 is different from most of the later cast iron engines. This feature was changed for the 1963 model year. The connecting rods were turned 180 degrees along their axis such that the squirt hole faces the camshaft. This applied to all Slant-6 engines EXCEPT THE ALUMINUM VERSION. Here the oil squirt holes point to the driver's side of the engine. This puts more oil on the cylinder walls. When the holes are pointed toward the passenger side, more oil gets onto the cam. Seeing that the Aluminum Block has an open lifter bore area, the Chrysler engineers felt that the cam would get enough oil from the top / rocker shaft so the Aluminum Block's squirt hole placement has always pointed to the drivers side of the engine.
To get satisfactory longevity and performance results for the aluminum Slant Six, extreme care and knowledge is required when refurbishing and building up one. The main bearing caps are an 8-piece set (4 uppers, 4 lowers, along with 4 locating sleeves and a 2 piece main seal cap) and must not be tampered with or installed out of order. Main cap bolts and head bolts are unique (these bolts are longer with more threads, see link). The Aluminum SL6 does not use freeze plugs or cam bearing insets. The cam runs directly on carefully sized cam journals, bored into the aluminum block material. In general, the block is more fragile and can be damaged easily, especially by corrosion, cross-threading or stripping out threaded holes. All the other parts used in these engines are the same as the cast iron SL6. External modifications (carburetor/induction, ignition, exhaust, etc.) pose no problems seeing that all the bolt patterns are the same. Be forewarned, anything that would tend to increase cylinder pressure dramatically (forced induction, tremendous compression ratio, etc.) is not suggested due to the already tedious head gasket sealing issue. Here is a look at a barrel which has been separated and cracked open due to extremely high cylinder pressures. Note how the iron cylinder liner is only about .100 thick and that it is held in place by an outer layer of supporting aluminum. (Click on any photo for a close-up view.)