One other thought...if you start with a 3.54 rear end ratio and your goal is to get back to a 12 overall first gear ratio (which will be what GM engineers wanted for a 1/2 ton truck with an inline 6 and what you'll have with the 3 speed and OEM rear end), you will need a first gear transmission ratio of around 3.40~3.50 or thereabouts. There is a T5 with a 3.50 first gear but it usually has a fifth gear of .72 or .73, so you might need to figure that in also.

~ Jon
1952 1/2 ton with 1959 235 | T5 with 3.07 rear end