I have an 87 T/an and I want to ditch the old 305 and put in a 350. But I have no idea what I’m doing and if I have to make any modifications to the engine bay. And will 4 speed auto work with the 350 or will I need an entire new transmission? Also the 305 is carbed. This is my first project car.

 

Transmission will be fine to keep. Make sure you go with an 87 or up 350, otherwise you'll need a different flex plate.

Otherwise, the swap is pretty straight forward

 

The 305 and 350, within the same year range, are IDENTICAL externally. Everything that bolts to one can be unbolted and bolted back up to the other. As far as year range, details matter; we'd have to know what years of 305 & 350 you're working with, to tell you which parts you need.

The transmissions in 305 & 350 cars of the same body style ("sportiness") are, AFAIK, IDENTICAL IN EVERY WAY. Bolt right up and work the same. Meaning, your trans will work just fine with a 350. Not "optimum" maybe, there's of course always room for improvement, but "fine" in the sense of not needing ... something unique to the 350.

Depending on the carb and intake manifold and the 350, you might be able to just swap all that over from the 305 to the 350. It'll all just bolt right up, subject to the 87-up vs 86-back intake bolt angle difference, which is easily overcome.

 

Other than possibly requiring a different flex plate, it's a straight bolt-in swap.