strength of the powerglide?
 

Now I have heard when it's modified it can be stronger than the TH series. I currently have a stock 3-speed powerglide in the car. I'm not sure how strong it is stock, can it handle a stock 400ci engine?

Powerglides are 2 speeds. They are easier to rebuild vs a TH. We've been 5.10s in the 1/8th (7.90-8.00 1/4 roughly) @ 142mph weighing 2900 pounds with a stock 1.76 gear set the only aftermarket parts used was the ultra bell, clutches, band, and valvebody.

If it is a 3 speed it is probably a th-350. The glide and 350 will interchange with on another pretty easily as far as the tranny mount and driveshaft length.

Either one would hold up fine behind a stockish 400, IMO.