This part of the P/S pump is the same.
The pulley (diameter and offset), the can, as well as the brackets will be different.
Some year /6 P/S pumps create less pump pressure (lighter car, less HP engine) – but I don’t recall the year frames. The reason I bring this up is some people prefer to have less power assist, and swap in a /6 pump for their V8. It doesn’t bother me, either way.
Once the pump is off car, the brackets can be swapped over. With brackets removed, there is a large nut (for pressure hose), that can be unscrewed. Once that is done, the can slides off. I would replace the can seals (large O-ring) for any P/S pump that has been removed from the car – for that is a very likely source of a P/S leak (or potential leak).
The steering gear are the same regardless of engine size. The /6 gears are fairly easy to change. V8’s with 3 cats – not so much.
The rear leaf springs will all interchange between all (’76-89) FMJ body’s – but there is a bunch of different spring rates. To make it easier, I use 4-leaf and 5-leaf springs (but there are variations between each type). 5-leaf springs are used in station wagons, police and vehicles with towing package – but not true in all cases.
The 8¼” will swap in place of most 7¼” - as long as you also have the shorter propeller shaft to go along with it (going by memory, but I “believe the 8¼” uses a 1.6” shorter propeller shaft).
The first couple of year 7¼” differentials used 2½” diameter tubes (instead of 3”) and the ISO-Clamps have different U-bolt spacing to accommodate the tube size difference. A huge majority of FMJ 7¼” differentials use the large tube (3” diameter) – so shock plates/ISO-clamps will interchange.
IF you glance at the differential tubes – where they are pressed into/welded into the carrier housing, the large tube 7¼” neck down, noticeably. Matter of fact, if you find any Chrysler differential that tubes neck down before they go into the carrier – it will be a 7¼” differential.
Personally: if you are going to have the differential out of the car – I highly recommend tossing the ISO-clamp system and either use an older Chrysler shock plate or use a kit from FFI. The car will feel and handle so much better. Also, the ISO-clamp is a weak link – that when it breaks – you will have to get car towed (tire touching rear inner fender and leaf spring will be touching the ground.
BudW