I would have been back sooner, but the site wouldn't let me on for a couple of days. I too tried the link and it wouldn't come up. I called them and got a message that Dan was ill and wouldn't be doing any business until at least April 1st. Hopefully he is OK by then and can help you.
Another popular 4-speed builder is David Kee. I talked to him on the phone and was very eager to answer questions. He said the T-10 (side loader) was something he did not work on because parts are no longer available from a manufacturing source.
Out here on the west coast, I called The Wright Connection and they told me they stopped working on T-10s as soon as all the parts dried up from "old timers" who had a stash. They were machining their own parts until the demand got too low.
Everyone told me the best source would be Ebay, Craig's List, or other internet search. I have seen a few side loaders at swap meets that were damaged and were being sold for parts.
I'm not a GM trans guy, but if some Ford and GM T-10 parts are interchangeable, I might have what he needs as I have a couple of broken GM T-10s.
Charlie Burns Laton, Ca (South of Fresno)
BurnsRacing981@gmail.com