I agree with you that a frame swap would be the easiest way to go. Converting the original frame to 4WD is always way more work in the long run, and when you need to get parts for it in the future, it's hard to get parts when everything is different.
If you use a modern frame, you can walk into any parts store and buy parts for that vehicle.
I've seen a 55-59 suburban that was put on an 89 or 90 suburban frame. I wouldn't go with the tahoe, suburban is a better fit. He cut the floor out of the 55-59, and put the full newer suburban floor, frame, running gear, etc. in the 55-59. It is quite a bit of work but it worked awesome, and this way he was able to adapt all 3 rows of seats, the carpet, and rest of the interior into the 55-59. Made it a 90 suburban that looks older. He says it works great!
