[70ragtop]

I am going to assume that the Firebird has a Pontiac motor and the trans in question uses the BOP (Buick-Olds-Pontiac) bell housing pattern. This being the case, the 400 is longer than the 350 meaning a shorter drive shaft. The mounting point to the rear is also farther back, but the crossmember in the Olds is adjustible and can be moved back. The bigest problem I see is the 350 trans uses a cable to kick the trans into passing gear and the 400 is electronic using a switch mounted on the gas pedal. If you can locate a switch and mounting bracket from a big block car (or perhaps your frind will throw in the switch from his Firebird) your in business.