Buy any newer motor you want. There is no problem. Once you get a motor and a trans that fits the motor, the rest is easy. Mock it up in there, using a universal tubular cross member motor mount and trans mount. When all done, have the drive shaft lengthened or shortened with proper slip joint/yokes. Piece of cake. You will work out any interferences like steering box, firewall, radiator, linkages and you will notice any other issues at mock up stage. Many folks have swapped motors into AD and TF and 60-66 trucks but there is no recipe or detailed instruction. Just be confident that there is plenty of room and plenty of help with any issue that pops up. You will be happier and have more options and parts available with a SBC donor.
I do realize you are on a budget but you seem committed to a modern automatic and a new long block or donor motor factored into your budget.
You are committed to new motor mounts and trans mounts already. It's a good plan. The cost is all about wheeling and dealing and DIY.
Buy the stuff, take off front clip and mock up. Take your time and sit there with the mock up and make friends with it.
I put a 454/350TH in a 56 GMC...........and added 4x4 to stock frame and body. It's all just grunt work and some fab. Mock up is absolutely necessary. Everything hangs in there on cherry pickers, blocks and jacks until you see all is well, motor is level and pinion angle is good. (broom handle)
The tubular cross member motor mount and trans mount eliminate a whole lot of fabbing and screwing around.
