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[TriumphR3]

Take lots of notes helps too.

[1project2many]

Welcome to the forum! It sounds like you've got a fun project.

You've got some good considerations ahead of you. 4L80E is a strong and capable transmission and it would be a good choice to use with the torquey 455. But it doesn't bolt directly to the Olds block so you'll need an adapter or two. 700R4 wasn't strong from GM but these days the aftermarket can often make them better. There's also a 2004R which was used behind the turbo V6 in the Buick Grand National. GN guys can make a bunch of torque and there's a ton of knowledge out there for building a capable 2004R. The If you are not likely to be towing trailers in the future the 2004R may be a good option.

One of the most common changes folks used to make with the 700R4 was to have the TCC disconnected. The TCC is the torque converter clutch and it locks the torque converter during low and moderate load so there's no slippage. Disconnecting the TCC on a stock transmission can cause overheating but folks really thought this was a good modification.

If the VSS were disconnected the ecm could get confused and run idle speed up too high. Do you remember that happening? If you look under the truck, the VSS should be located at the driver's side rear of the transmission. You can see if it's present and if there are wires connected to it.

Dave W has offered really good basic steps to get started. Tunerpro is a great tool and it can be useful for basic diagnosis of problems as well as for tuning.