Good on ya for being patient enough to solve the problem. Any trans the size of a SM-420 or bigger definitely needs some sort of support as you're installing it, for safety reasons as well as ease of installation. Here's a little trick I've used successfully all the way up to big rig trasnsmissions- - - - - -put a long piece of heavy wood like a pressure-treated 4X4 landscape timber, or even a 6X6 through both windows, padded with rags if you don't want to scratch up a nice paint job, and hang the trans from a cable come-along. You can make very accurate height adjustments by using the ratchet on the come-along, and there's enough fore-aft swing with the cable to let you stab the input shaft safely.
Jerry
