jonota wrote:
the other concern with the 4T80 is a relative lack of durability
Well, kindasorta. The usual problem is the soft shift. That puts a lot of heat into the clutches, which get glazed and then start chunking.
You can do some tweakery with the ECM programming to firm up the shifts, and if/when the transmission needs service, it will respond to the usual heavy duty modifications - reducing the clutch pack clearance, some oiling improvements, clutch materials, etc.
I've done several T350s, including the one in my daily driver and the one in a friend's big block Impala drag car, with over 400 passes on it now. I'm not a Real Transmission Expert, but it's not rocket surgery.
A while back I had plans for a build using a 4T60, bought the service manuals, and got familiar with their innards. The T series boxes have a chain and the differential, but otherwise parts is parts.
*If* the transmission fails somehow *and* you don't just replace it with a junkyard box, the transmissions are modular and come apart in easily-serviced chunks. You need to make a few compressors and pullers, a few minutes' welding for a Locost builder. Most of the bits only go together one way, and nowadays a digital camera will let you document each bit and where it goes.
I might be going on a bit, but I avoided transmission work for years, blinded by the FUD of looking at complicated hydraulic diagrams and pictures of what looked like thousands of gnarly bits. It's nowhere near as complex as it looks. Trust me, if you can weld up a Locost chassis, you can rebuild an automatic transmission if you ever need to.