my 67 firebird has super turbine 300, will it switch out with a 350 turbo similar to the powerglide. also which is the best transmission for this conversion

My understanding is its a direct swap. Converter, cooling lines and driveshaft should all work. Any BOP th350 trans should work.

I did that swap (ST300 out and TH350 in) about a month ago and it went right in. You only need to use the TH350 converter and get the bracket for the kick down cable (which i would use).
This is a good swap even though both transmissions are 1:1 final ratio.