I would say go era correct, so stick with 69 and earlier, it may initially be a pain, but once you have all correct pieces it will work perfectly. I may be be wrong but chk the 68 340 rad, I recall there were 2 different specific #'s. Auto vs std(cooler lines vs no lines).
Repop new wp are available for 69 and down. Problem with them is the bosses arent machined 100% to oem spec(mine wasnt anyway)
The big question to me is if you have a 68 radiator #2898026 that fits your 68 fan shroud would you swap to pre-70 water pump. It's almost like you have to.