Need suggestion for rear end replacement

there is a company called MAE Racing that rebuilds and sells all types of rearends. They have a store on ebay, probably have a regular website too. I'd go for a rebuilt stock differential with a Ford trac-lok and whatever gears you want. A lot of people push home brewed 'posi' setups, stick with real ford stuff though. The trac-lok is a limited slip that uses clutches and is rebuildable (unlike the Auburn).

http://cgi.ebay.com/ebaymotors/FORD...026229527QQcategoryZ33731QQrdZ1QQcmdZViewItem

The rearend (67-up 8") referenced by 302 Coupe is the minimum you should consider. I don't think that they would recommend that rear for a 400hp 347, however. The '67-up 8" is better than the earlier 8" rears, but not really up to the task of handling your motor if any spirited driving is done. I have heard a rating of 350hp for this rear, but that is only an estimate and depends of several factors (clutch, tires, to name two). You should consider a complete 9" assembly with 28 or 31 (better) spline axles. That would be fairly bulletproof. Currie makes several that would fit - I just installed a Stage II Musclecar rearend in my '66. It fit well. I am using the TrueTrac diff, but there is nothing wrong with the Ford stuff, except it will require rebuilding at some point. The TrueTrac doesn't require rebuilding, ever. Others can offer a similar unit.

I met a guy with a 331 putting out similar power who broke his 8" pinion support a couple of times before going to the 9" and he only had 225/50-16 tires. I don't know how hard he drove it. You may have better luck. I just want you to think about it.

Start looking at the 9" units for that hp (better safe then sorry). Also, the 9" center units are fairly common with many aftermarket products.

The only issue I see for a '66 is the smaller flange to flange distance in your axle housing (differs from the 67-70 and 71-73 rear ends). If you like to run the more common 7 or 8+ inch wide tires and don't want to flare the fenders, try to find a 64 1/2 through 66 Mustang or 57 through 59 Ranchero 9" rearend.

I'm putting a 9" from a 59 Ford sedan in mine right now. The 58 and 59 sedans will also work. The spring width and total axle width are the same as early Mustangs.

there was a discussion about this on fordmuscle.com and 80% of the people with 400+ HP havent broken a 8" and stick with them. If your not launching on slicks at 4000 RPM,a 8" should be fine since your not shocking the rear.For a 4 speed,no more then 3.55's and that might be a little high for freeway speed with no OD.

Most people with 400hp, don't have 400hp. I would not start on a dead end road. People say I won't drag race it, or never with sticky tires, or never put a power adder on it, I know I've said them. Only to go home on a trailer when I tried all 3. I would hate to build a good motor and trans, and have to baby it because of the rear end. Build it right once. With an 8.8. Or a 9".

at 400hp, especially if you are going to run slicks at all, you are on the edge of the 8" rears power limits. you can beef the 8" up a bit, but i would be looking for a 9" rear, or consider swapping in an 8.8 rear.