trans question

hi everyone just joined this forum first i would like to tell everyone a bit about myself i am 22 yrs old in the military and love muscle cars my goal is to have a car that looks nice enough to be admired by ppl that see it but also have enough bite for ppl to say wow check that out...i have a 1977 nova swapped the straight six out with a 455 olds motor bored to a 462 currently i have a turbo 400 backing it my question is this... with performance AND mpg in mind because i do drive it regularly should i just change the gears with a shift kit and mini spool or would i be better off putting a 4 speed in it or a TKO trans? as of now doing 65-70 mph i get ROUGHLY 12 mpg hwy which is not certain considering the speeds vary as do throttle control however i WOULD sacrifice to mpg for power... no matter which trans i do i will put a mini spool plus a shift kit in my car and possibly change my gear ratio if need be to get what i desire which is a street car with enough power to have fun with... the other alternative is to put a 6 spd in the car but i think that would kill my mpg considering i have a lead foot...this thread is mostly just to see what others would do before i make a decision

here is a picture of my car it is all stock except motor trans and exhaust

http://i1185.photobucket.com/albums/z352/naproxin420/IMAG0018.jpg

i have another question also does anyone know the max tire size without tubbing or anything of that sort?