4 wheel drive question

[1972BackInBlackC10]

iv been looking at a 1969 chev 4x4 the owner says it has a np203 transfer case if i remember correctly, anyways he said the truck is full time 4 wheel drive and you can only do 80 kmh

the speed limit on our highways is 100 kmh not that i would be doing much highway driving with it even with that limit in town the truck would possibly be reving higher than it needs to im assuming?

my main question or concern is obviously if i bought it id want to change out the transfer case so i can drive it properly and disengage the 4 wheel drive when its not needed, what transfer cases are useable on these trucks as in what years can i go up to without modifying anything or much, these trucks in 4 wheel drive arent very common around here surprisingly enough so if the transfer cases are 67-72 specific id be sol