69 firebird needs new trans... need input

[Grambo90]

hey everyone, i have a 69 firebird with a pontiac 350 with a 3 speed slushbox... no fun, especially at the pump. I want to drive it everyday, so im going to rebuild the engine back to original specs except for a mild cam because i want decent mileage. So my question is... what transmissions out there will fit the poncho block and the trans tunnel (i dont want to cut the car up) i was thinking the cheapest route would be a 460L-E, but what about a t-56 or a tko-500. any help is greatly appreciated.