I am new here. On my way to a cruise here in Oregon I developed a rod knock (bad)and had my '66 Valiant Signet towed home. I have a chance to get a used 225 /6 engine from a '78 or '79 Volare at an engine shop that I found. The question is... will it go right into my '66 as it is. I've had enough trouble finding motor mounts that work for the Valiant, let alone a whole engine. Any input would be very much appreciated. Thank you

The back of the crankshaft changed on all Slant Six engines in 1968 but your proposed swap will work if you do the following:
You need to use the flex plate / flywheel off a 68 and newer SL6.
You will also need a small adaptor ring sold by Wildcat Wrecking. http://www.wildcatmopars.com

You need to swap the oil pan and pick-up tube off the 66 engine and onto the later SL6 so it will fit into the 66 Valiant chassis.
DD