59 American t5 conversion?

In 59 you only had the L-head, IIRC. With OD those had like a 4.10 gear, I think 3.08 with standard trans. You really need a 3.31 or 4.11 to use 5th gear, though 4.10 might make first a bit short. 3.78 was also an available gear, and might be a great choice. You can vary tire diameter a bit to alter final rpm 100-30, depending on how far from stock you're willing to go. You MUST use at least 15" wheels. Some of the older thicker cast aftermarket wheels will cause some dragging of the tie rod ends. Chryslers use a 16" steel wheel, but they are deep -- you'd need spacers. But running a 16" wheel makes it a lot easier to find tires in stock. You can get just about any 15" you might need, but the tire store may have to order them. Keep a good spare if you have to order a special size tire -- a full size spare.

If you're keeping the L-head you will need a lot steeper rear gear to use the OD. 3.78 or better -- they came with 4.10 (or 4.11, depends on how you round...) when using the T-96 w/OD, which was a 0.70:1 OD (all the BW units were that ratio). Otherwise you will have a great four speed trans, which is still a big improvement over the T-96. First gear on the T-5 might be really low if you use a 4.11 rear axle. There might not be an easy way to go without having either a really low first or a really high fifth. A first gear that you only use on steep hills and/or with a heavy load might be better than a fifth you can only use over 75 mph...