Rule Number One: Never throw anything away. But now you know that!
As I remember, there is a radiator for the 1975 and then a much wider radiator for the '76-'79. I do not remember there being a change in the measurements fore and aft. So, either the fan/water pump are incorrect, the radiator is incorrect, or the cowl is incorrect. You can contact Glenn Lenhard of Glenn's MG Service in Tampa (he's closest to you) and ask / travel there for an inspection. You can order the correct used cowl from Paul at Sportscar Craftsmen in Denver CO -- maybe even a good used rad.
The earlier Spridget engine uses a completely different water pump and fan, so that's not a solution. Some of the Triumphs used a fixed fan (without the clutch), but I don't know about the spacing on those. If you sent me a couple of pictures of what you have in place now, I might be able to make a suggestion. I don't have a 1500 Midget close by to inspect. Let me know what you eventually find -- or how you have solved the problem.
John Twist