Not to belabor the point:
B and E body cars used the same (size) radiators in 1970 and 1971
Factory air conditioned cars did have a higher capacity radiator from the factory for slant six engines
B and E bodies had two different physically sized radiators;
19", #2998936 for the base 198 and 225 ci motors
22", #2998937, slight up-grade used on AC cars in both manual and automatic transmissions.
Not sure what you might have currently?
That being said, I would think that your car, if it came w factory air should have the 937 radiator if it is the original.
Another suggestion could be to take your 6 cyl radiator and have it re-cored with extra cores to enlarge it's cooling capacity. This is a common modification on many cars. My one thought was that it would be easier to have a radiator modified rather then going through a lot of extra work to weld in a new rad frame.
Of course if you are looking to install a V-8 engine later on, it would be worth going thought the effort and expense of welding in a new frame.
Another option would be to score a 22"A body radiator from say a 73-74 slant six cars with AC
3673005 was a 73 A body radiator w AC- 22" unit.
The cooling capacities were increased in those later years due to the engines running hotter because of the increased emission controls.
It would your decision based on your needs, just wanted to give you a larger picture.
Just thought it is easier to modify or change a radiator, than to weld in a new frame. (If your welding skills are better then mine- go for it) That's just me.
Good luck with whatever you decide to do.