’73 Plymouth Duster Help

Hey everybody, i've been trying to get myself a sweet muscle car for a while now, and i've recently came across a very promising looking Duster, not to far from where i live.
"1973 Plymouth Duster. 225 slant 6 engine, auto tranny, CD player. Runs good. Daily driver for son to school. Bucket seats. Rebuilt carb. New front end bushings, tires, heater core. Pretty cool car. Probably never see another like it."

This would be a summer/winter father/son project, and i think a promising first muscle car. I had looked at Chargers for a few years and just had a hard time dealing out the cash people wanted for questionable quality.
I've yet to go visit the car and test it out, and i stand at 6'6'' and around 250 lbs, and was curious if you all think i'll even manage to fit into it? Inseam of around 38

Also Dad and i are wanting to drop a 340 into it, or possibly a 383 and had concerns over if the tranny or rear end would need to be upgraded. I read that the 73s could come in 340 or the Twister, and until we get ##s from the car, i have no idea what it has but we believe it is all original.

Thanks for any help, we appreciate it!
Author: Icarus