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As BR3W said, the mega shifter is really easy to install, I used one for years and I think it took less than an hour to install and adjust up.

Your bigger challenge will be your existing bellhousing crossmember and getting a new transmission crossmember. You mentioned having a compound low, which means you probably have a Muncie SM420 transmission which doesn’t use any transmission crossmember mount, it just hangs off the bellhousing. The bellhousing has its own unique crossmember which does not unbolt from the frame (at least not on my 65).

I’ve never done a manual to automatic conversion on one of these trucks so hopefully someone can describe what needs to be done to remove that existing bellhousing crossmember and what a good donor vehicle would be for the transmission crossmember. Also you’ll probably have to modify your driveshaft to get the right length.

Oh and I think the starter will need to be changed as well, because the existing one probably bolts to the bellhousing instead of the engine (at least mine does).

Not the best of news, but it’s better than getting under there and realizing there are more parts you need to get it finished.