Try finding an aluminum radiator that is large enough that is cross flow and convert it to down flow (I expect part of your problem is that you need a tall & narrow radiator).
I did that with the one in our Anglia. It's a Howe Dirt track radiator, weld on a new filler neck and a bracket to hook the brace rod to (and fab a lower mount) and Voila, you have a radiator that will cool great and shouldn't break the bank. I chose the Howe (got it from Summit) because it has no epoxy in it and it was cheap (just over $200 back then) I farmed out the welding.
If you try this make sure that whatever angle you are going to install the radiator at that you have the filler neck (if it's not directly on the radiator) feeds at a downhill angle into the upper tank. If you don't it will always have coolant in it (making it dificult to tell if the system is full) and is a pain to fill as you have a built in flow restriction there.
Hope that helps.
