64 short bed transmission ?

[otiger50]

I'm 19 and I have pulled and rebuilt the engines from a couple of early 2000's Chrysler cars and I am a full time paintless dent repair technician, but other than that I am inexperienced in every area. Therefore, I am seeking help and advice on my new project pickup.
I just recently bought a 64 shortbed. It has a small block but I am unsure of the exact cubic inch. It also has a 3 speed in the floor. The motor misses sometimes when you get on it, and the transmission won't stay in gear.
I would like to pull the motor and trans and do an overhaul to the engine and possibly do some mild tuning such as a cam, a different carb (a 600 cfm edelbrock is what I have in mind,) headers, and possibly an MSD ignition. For the transmission, I would like to scrap the 3 speed and bolt on a T56 6 speed.
I would just like some general input and advice before I start in on the project so that I don't get in over my head.
my MAIN CONCERN is putting in a different transmission