this is a good site to compare tire sizes. it has a few tabs for other stuff as well like wheel offset calculator
https://tiresize.com/calculator/
this one talks about scrub radius changes with different offsets.
https://www.wheel-size.com/calc/
for a rear axle ratio I personally like the 3.73 ratio if you have an O/D trans, which I assume you would
the "new springs from a different truck of about the same weight" only makes sense to me because you can change the heights of the frame mounts as you see fit, after the truck is together. parts would be readily available because they come from a truck that is fairly current as far as parts go. with that said, be aware that the front suspension cross member would need to be installed at the rake angle that you want to finiah off with so decide before hand what that will be and if you will use different outside diameter tires front to rear. a bolt in front cross member also makes sense to me because a weld in member, once welded in, if it isn't right you are gonna be stuck with it as is or be in for a bunch of careful cutting if you wanna re-use the parts. I highly suggest you read up on independent front suspension geometry before you do anything.
planning mode is great to help reveal what you really like and then if you post those wants the guys here will come up with lots of ideas on how to get there before you start anything.