I would agree that a frame swap without plenty of welding experience could be too hard, but there are other options. If you swapped out the crossmember you could convert to the 5 speed you want. I'm not sure what all you can do to convert to disc brakes, there may be kits available. I do know you can convert to power steering/brakes by using early to mid 70's parts with only slight modifications.
For the trans, 4.03:1 first gear should work just fine. As far as cruising goes, that depends what your 5th gear ratio is, as well as what gears you have in your rear end and what size rear tires you have. But as long as your final gear drive on both transmissions is close it should be fine. Does the 3 speed you have now have overdrive?
