400 big block - a883 od

There is no way I would put that trans behind a 400.
Here's why; even a 318 can blow up that wee small 18 tooth M/S overdrive gear.
You will be tempted to put some performance gears in her to make use of the overdrive . Am I right. Maybe 3.91s, yeah I bet. Ok that's all good but when you get to 65 mph, in direct and you pull the stick, you will be going from 3160 in Direct to 2300 in od. Saaweet!. Ok that's all cool ........ if you remembered to take your foot off the gas, and ease into overdrive, else at 3160, your 400 has enough torque to blow that tiny wee gear to smithereens.
Fine you say, I can still limp it home in direct. HaH! When that od gear blows up, it shatters into bits and pieces and you have no idea where they land. Chances are excellent that they lodge on the cluster and the instant you let the clutch out, other gears will follow suit and rip teeth off.
I can tell you that with 3.91s that 400 will be a blast to drive, and there will come a day when you forget that your 4-speed is NOT a 4-speed at all, but just a 3+1, and when you slam it into "4th"..... Ziiiiing ...... and then you will remember,lol

OK wait aminute; If you can figure out a way to lock overdrive out, long enough to think about it, then you could use it, just don't floor it,lol.
OK here's another tip; a 400 is fine with hiway gears to start with, so how about 2.94s and a regular 4-speed. With 28" tires 65=2300, badaboom.
2.94's! Well you got a clutch don't you; on the street, they rarely blow up; rev it up and go!
At 32 mph in 2.66low, your rpm will be 3000 rpm. Hammer down and light 'em up. Just don't copy the factory low-compression slug engine build.