250 I-6 question

[ItalOso]

Hello Members,
I have a '64 GMC, SWB with SM-420, Fleetside 1000, rebuilt the original 230 to a 250 over 10 years back. Thinking of different things to do, but not sure where to turn. Had received a link from another member to adjust/rework the pedals so they aren't so high up off of the floor.
I'd like to consider putting a newer manual OD transmission into the truck but not sure what will work. Realize the driveline will most likely need to be rebuilt, do these tranny's just bolt right up to the truck? is there a housing that would need to be added, thus placing the stick back farther than its stock location now?
I have changed the rear end gears, which my grandfather had set up for hauling, like 3:93 but I reduced to 3:05. I've noticed that I have to really rev up the motor to get the truck moving as compared to before, it took about a 1/4 of the power with the old gears.
If I choose an automatic, like a 700R-4 is an option, would this be the best choice? I'm concerned if the 250 will have enough power to provide to these new transmissions. I will change the steering column and reduce the steering wheel diameter too depending on which choice I may make.
I also do not have power steering on the truck now, but increased the wheel size from 15 to 16" because of the addition of power disc brakes a couple of years ago. Seems to turn easier with the bigger tires but with the reduction of the steering wheel and alterations, should I consider just installing it at the same time?
Appreciate the feedback and links/pictures of the changes that have been made. Thanks!
ItalOso