I have a 87 pontiac firebird I am replacing the engine with a BP small block 400 rate at 508 horse power 473 ftlb what will be a good transmission to hold up to 500hp that will fix in the 87 firebird it a 5 speed manual transmission woul like to put in a 6 speed if possible

 

A magnum t56. They make an f body version.

 

Thank firechicken82 you wouldnt know who might sale the t56 transmission

 

There are MANY places. Some of them even have entire "kits" for these cars. (or at least, did, when such things were more popular than they are now, as the cars have disappeared)

Google is your friend.

 

Hawks, tick performance, bowler transmission.