M-body 8.25 posi to a 66 A-body questions

[66durgederp]

So i posted this in the early a-bodies forum but thought afterwards this would be a better place to ask, sorry about that. Im just checking myself here and would like a couple pointers if needed... So yesterday was gran fury destruction time. surprisingly it came apart like butter. searching through numerous threads i found the measurements of the posi 8.25 from the fury, vs my 7.25 posi currently in the derp.

8.25 Dippy/Fury "M" body:
Flange to flange- 54.34 inches.
Perch center to perch center- 44.46 inches
Center line of axle shafts to centerline of u-joint- 11.69 inches

7.25 derpadurp early "A" body:
Flange to flange- 53.2 inches
Perch center to perch center- 43.02 inches
Centerline of axle shafts to centerline of u-joints- 10.09 inches

so to shorten the driveshaft i would need to remove 1.6 inches, right? or is there some drivetrain geometry trick or calculation or driveshaft measuring anomaly im potentially missing?

I know id have to move the perches in .72 inches per side as well, however my biggest question is, since my 66 hasnt been mini tubbed or anything, would it be safe to say if a 15x7 with 4 inch backspacing fits like a glove for a stock setup, if theres .57 inches additional per side, then a 15x7 with 5 inch backspacing should be the cats meow?

Or are the extra .57 inches per side going to cause some headaches? thanx guys!!

 

[Hersbird]

That sounds closer to the a body axle, I thought you were saying it was 1.5 inches per side wider. Although you might want to measure the wheel mounting surface to mounting surface because if you are using the brakes from the cop car there might be a little more spacing there as well. This thread seems to say 15x7" with 4.25BS leaves 1" clearance, so a 5" might fit pretty well. If you go over a bit with backspacing you can get if back with a spacer while to little back space will take away some of your room on the outside. http://www.forabodiesonly.com/mopar/showthread.php?t=186503

On the driveshaft that should be right but you might want to get it all together and measure it first to check. Are the u-joints the same size?

 

[Oklacarcollecto]

That sounds closer to the a body axle, I thought you were saying it was 1.5 inches per side wider. Although you might want to measure the wheel mounting surface to mounting surface because if you are using the brakes from the cop car there might be a little more spacing there as well. This thread seems to say 15x7" with 4.25BS leaves 1" clearance, so a 5" might fit pretty well. If you go over a bit with backspacing you can get if back with a spacer while to little back space will take away some of your room on the outside. http://www.forabodiesonly.com/mopar/showthread.php?t=186503

On the driveshaft that should be right but you might want to get it all together and measure it first to check. Are the u-joints the same size?

You as stated you need to measure outside drum to drum. The newer cars have wider drums and that seems to make a difference when upgrading an a body 8 3/4 to large bolt pattern by requiring longer axles than original. Some of the backing plates and drums are comming off the M body rearend you are talking about.

It is very important that you get the perches in the right spot on the tube because if they are too far forward or backward it throws everything out of sink resulting in vibrations from hell.

 

[66durgederp]

perfect. yes the u-joints are the same thankfully! Also since its a cop car it has the 11x2.5 brakes instead of the 10x2.5's. ill get some measurments as soon as i can. thanks for the tips guys!