OBS chevy LSx wiring

[haveTBIwantLS2]

How are you guys running the two harnesses? What about ECS controls in the cab? A/C, tach, all the gauges, etc. Is everybody keeping the old computer hooked up and running the 0411 seperate? I'm primarily concerned with the speedo and tach.
I guess I just don't get how the two harnesses work together. I'm on a Macintosh right now, so sorry if the text is messed up...
Thanks.

Can I just remove the old harness entirely? But then what about the tach/speedo...ughhhh

current setup is GMPP350HO, TBI (90 lb inj's), AFPR, all the good stuff. DIY tuning

so guys are running two isolated harnesses, huh? One for the powertrain, then one for the chassis?

I bring up A/C because of the computer (7747).
I'm aware of standalone engine harnesses and that's great...but what about retaining my 1991 gauges and controls?
I hate macintosh computers

the only real thing I think I need to know from the LS1 computer is check engine, tach, speed
A/C switch too.