I'm getting a 8 3/4 for my 71 dart with a 904 in it. My question is will the drive shaft for the 7 1/4 work with the 8 3/4 rear? In my Haynes manual it says they are about a 2 inch diffrence. The new rear has a 5 on 4 bolt pattern which is perfect for my alloy wheels. So it should have 9 inch brakes. Will the brake lines and parking brake match up? And is there anything else I should be aware of before starting this project. Also what is the best gear oil for this?

The pinion on a 8 3/4 is 2.26" shorter then a 7 1/4 so you'll have to shorten the driveshaft accordingly. If you don't the can run out of room to slide in the trans during suspension travel. You don't want that to happen. Heck you probably have about that much in normal travel so if you don't shorten it you probably will put the yoke right up against the tailshaft.

Sounds like you are getting a native A body 8 3/4". It will not have 9" brakes, it will have the oddball 10 x 1 3/4" drums. Yes parking brake cables and flex hose will be fine. Will the 8 3/4 comes with the hardlines? Brake parts may be hard to find - you can use a 71 plymouth duster 340 as an application that rear end came with stock.

Edit: you'll need the 3" U bolts and shock plates - 8 3/4 (and other mopars of the era) used 3" axle tubes, the 7 1/4" used 2 1/2" ones. I recall reading some have got away with egging out the holes in the shock plates but be careful make sure there's enough meat left.

Ok so I can shorten the drive shaft, that's easy. The brake lines will not be on the new axle, will the old ones interchange? And what about filling the old holes on the shock mounts and drilling new holes.

you are going to need the larger 8 3/4 shock mounts and u-bolts. you may be able to egg out the holes but they will be close to the edge. imo its not worth it. no idea on the lines themselves. they may. if not you should be able to reuse your splitter and just run new lines.

Firmfeel has the shock plates for $125 + shipping. They're nice and IMO worth the money, but if you want to go cheaper, you can try hitting up car-part.com or your local pick-n-pull. The U-bolts, on the other hand, you can get anywhere for a hell of a lot cheaper than the $50 that Firmfeel wants.

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Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

I recently got two zinc plated 3/8-16, 3" x 4 1/8" U bolts from online bolt seller BoltDepot for $6.68

they have stainless for about double that.

Shock plates from any 8.75-equipped model (A, B, or C Body) or any car model using an 8.25 will work. I'd either fabricate new lines or order the pre-bent lines from Firm Feel or similar company. If ordering you'll need to know where the vent tube bolt is located - early 8.75s are close to the pig area whereas later 8.75's were located out on the pass side tube.

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65 Dart station wagon slant 6 - now under construction
47 Dodge Custom 4 Door sedan
87 D100 Short Bed slant 6

Retired USAF 1966-1986
Retired US Postal Service 2004-2014

Other random note - The screw that holds down the left/right splitting block is a special vented piece, part# 4032798 that can be had from ebay or your local dealer for msrp. 8 3/4's don't have a built in vent. Also got the shock plates on ebay for $60ish.

ESPO had the u bolts for $3.xx each last year. They should be 1/2" ones not 3/8. I bought 3 sets... even the local parts houses wanted $12+ each.

If the lines will work they will need tweaking. The wheel cylinder will be a bit higher on 9" vs 10" brakes. The center loop they go over to clear the gears is bigger. I was forced to fab lines as I used disc brakes. A small set of lines like this is good to learn on if you haven't made them before but will require investment in a good flare tool and bender. Learning to flare is an art within itself.

correct, I should have mentioned the ones I bought were for a rear sway bar install, not for attaching leaf springs..just trying to highlight that u bolts are available from many sellers

Another random thought - drill and tap a 1/8" npt plug on the bottom of the housing for a drain. Use a recessed allen plug so it won't interfere with the jack. Makes future dropout changes less messy.

The cheapest plates I found were 125. So I bought a u bolt and plate kit for a trailer and I'm going to cut and weld the shock mounts on those. Save my self $100. And it looks like I can make the brake lines fit just fine.

I also need to weld on new spring perches because someone tried to use the rear for a rat rod and welded on makeshift ones. What is the distance between the inside of the drum back plate to the edge of the perch?

I can measure mine to be certain. Thischart may help to some degree.

Spacing is the easy part to get. Angle is going to be tricky. This sort of thing is usually done on a jig.

Yea I need somewhere solid to measure from so I can place the perches. As for the angle I thought I could just level out the whole rear end on stands, then put a level on the perch and make that level or parallel too the rear.

Using the axle flange is probably the best reference for distance.

I can't picture what you mean by the levels but as long as you can assure the pinion angle relative to the perch angle is proper then your set. You can use your old rear for reference on angle but that won't work for distance because the rears have different overall flange to flange widths (probably made up by brake width and offset to get the wheel in the same spot).

Distance has to be as close as possible but you have some wiggle room with the angle because you can use wedge shims to adjust.