A couple of things that need mentioning.
The transmission crossmember on FMJ’s do not have torsion bars attached to it – so that is not as big of a deal as the ABCE-bodies have. This is from a ‘68 (or ’69) B-body and in the Left lower corner, you can see the rear torsion bar attachment. Modifying the crossmembers in those are a bit trickier sense there is a lot of stress involved.
FMJ bodies transmission crossmembers do not have that added stress – so a person has more room to, um, make changes.
There are pictures on modifying the crossmember on this and a few other threads on this forum board
42RH Swap: '80 Volare .
These four pictures are from forum member mchartier105
These two are from 80Mirada
The next one is from 76Volareman
Basically, the crossmember (the part that is welded onto the floorpan) is roughly 2”x3”. In this case, portion of chassis will contact the overdrive portion of the A500 (or A518). What some members have done is cut away about 1” of that crossmember (or about ½ the thickness) and welded in to fill in that box – once it has been determined there will not be any contact between the two items.
The second part is the removable crossmember needs to be modified because the transmission mount is different (the old style (original) mount will not directly attach to transmission). This part is not too difficult for someone with some metal working and welding skills.
Sense this is not a critical or stressful location, I think trimming the channel to fit then filling in with similar thickness metal, then painting/rust proofing is all a person needs - except for high horsepower and/or racing purposes. This is what I plan on doing, when the time comes.
Others have cut a portion of floorpan away and welded in a new box channel which replaces the existing one. This method I would use if torsion bars make contact to this crossmember (like ABCE-bodies) but a bit of overkill for FMJ’s (unless for high horsepower and/or for racing).
Forget about the OD. 8.25" rear axle from a cop car with 2.94 gears. Put a 360 Magnum in front of it with the factory fuel injection. Have fun.
All in all, this might be the best answer. 3.2 gears are better than 2.9 gears but 2.9 is still a decent ratio. It is not too hard to find 3.2's for the 8¼” differential. 3.5's are even easier to find (common for pickups and vans).
If a person was to put 3.9's into an 8¼” and also go with overdrive, you would have a wonderful combination - but is a bit of work to do.
On a different subject, I have an ’84 M 8¼” differential with 2.94 gears that I might sell. I purchased a limited slip carrier to put into it. This spring I was going to replace all bearings, install the limited slip and toss it into my ’86 Fifth Ave.
That said, I just purchased an 8¾” differential to put into the car (for the big block I’ll be installing), so technically, I won’t be needing the above mentioned 8¼” unit and might be sweet talked into selling it. It is currently in Oklahoma City.
Ironically, I just drove through Austin TX a couple times this past weekend.
BudW