DBW is tunable. GM has it setup with some lag in the factory tunes, but the tuned trucks I've driven it's instant.
I don't believe ABS and traction control would be an issue, but just let your tuner know you won't be using it.
An older t-case should work. If it were me I'd go grab one out of a 90's 1-ton with a manual shifter. You could probably find an adapter kit for your original one from advance or novak if you wanted to keep it also.
88 RCLB: LS swap in progress
00 CCLB
03 Yukon Denali: Wife's "soccer mom mobile"