The FMJ axle is a tad narrower than the '64-'70 B-body rears. The '64 you'd want to avoid because of the hub design (do you own a drum puller? Probably not). The '65-'67 rear is actually narrower than the '68-'70, but either way you're good for inboard clearance as long as you don't go too far beyond the factory 4.5" backspacing used on 7-8" wheels. The spring perches distance is only .44" different, which is .22" per side... you can literally pull the springs into place. There's not enough bushing bind to worry about if you have stock springs with the oval front spring eye. Aftermarket springs don't have that, but it'll still go with a little muscle. It's a pretty straightforward swap that can be done in an afternoon if you're correctly prepared for it.
A couple of things you'll need to know:
Your driveshaft may need to be shortened a little for the 8.75" rear since it's longer from the axle-shaft centerline to the input U-joint flange. I say "may" because I've gotten away with the OE driveshaft a couple of times, but you absolutely want to check clearance between the front U-joint flange and the output seal on the transmission. If it's compressing the factory boot on the seal with the driveshaft, you should probably have it shortened. A little bit is OK, but expect the boot to wear quickly.
The B-body axle never came with iso-clamps prior to M.Y. 1973 (the '71-up axle is way too wide). You'll either need to A) use A/B/E-body lower shock plates or B) use a set of iso-clamp adapters that will mate the small pin of the B-body rear to the huge pilot hole in the spring clamps. Option A is the preferred route, since you lose the sloppy iso mounting of the rearend, plus gain a better shock design in the process (use shocks for a '66-'72 B-body application). The spring plates are available as reproductions, or less-expensive alternatives can be had from Speedway Motors. Option B requires finding those adapters (with a different center bolt) and using stock everything else. Mopar used to make the adapters, but they've been discontinued forever. I have a couple of sets of them around here somewhere; I never used them because virtually every time I've done this swap, new springs went in at the same time and we used B-body shock plates & shocks. It's simply a far-better setup.
You will need to rework your brake lines & hose. The brake lines are pretty simple: order a set of reproduction lines for the axle you're using, with the brakes you're using (if they ask). The brake hose is a different kettle of fish. To do it correctly, you'll want to find the hose for the rearend application (not your car), and then have a competent parts man open a book and find one with the same ends, but with a hose a couple of inches longer. This means Advance, AutoZone, or O'Reilly's won't be able to find it since A) they don't generally have competent parts people, and B) they almost never have books. No, you're not gonna find it on RockAuto, either. Done this way, no modifications to the body-side brake system need to be made whatsoever. Well, after you've twisted off the original brake line trying to get it loose from the hose connection. That's a given up here in rust country.
I've never tried the 8.8" Ford swap, but I tend to shy away from that rear because I've sold so many repair parts for them... sometimes they go forever, other times they go bang. They were used in some pretty stout applications, obviously, which is a good testament to the design. On the other hand, when you're constantly selling bearings and gears for F-150s because the rear axle failed, you tend to form a negative opinion. Just my two cents.
As far as putting in another 8.25" Chrysler, my thought is simply that if I'm taking out an already-weak rear axle, I'm not replacing it with an equally-weak one. I'm not a fan of the C-clips in that particular axle either. After watching a LH axle shaft walk out of a gorgeous survivor '73 340 A-body at 40MPH and destroy the entire driver's quarter and taillamp panel, I swore off the 8.25" for performance use forever. We were driving alongside him at the time and he just couldn't get to the side of the road quickly enough. However, lots have had good luck with it and it's a direct bolt-in, but if you have to have the cover off for any reason, I would recommend replacing the C-clips with new ones.
