Will an 85 stock 700r4 handle an engine in the neighborhood of 300-350 horsepower?

 

You really should look for an 87 to 93 transmission if you're replacing yours. The internal updates make them a bit more reliable.

You will need a companion torque converter as well. Yours uses a 27 spline piece like a 200 and the newer models have 31 spline input shafts.

 

Yeah, I've been hearing that the newer ones are better trannys. Will they be able to deal with that amount of torque okay though or will i need to put a kit in it?

I'm also not so much replacing it as making sure it will work. i pulled this motor and tranny out of the same car, and before i finish dumping a couple thousand dollars into the build on the motor, i want to make sure i'm not going to blow the tranny up right away.

 

The original 700r4s like yours are not good behind fresh engines, especially when it is probably well used to begin with.

Now is the time to upgrade. I wouldn't even rebuild yours; there are too many upgraded parts in the newer ones. Sell yours on ebay if it works and go to the junkyard to get a newer one and have it rebuilt. Or, you could use yours as a core charge for a good aftermarket HD transmission. Ebay and the internet are full of those.