SD7H15HD-FLX worked well on dual system on 91 suburban for me. Parallel flow condenser and elec fan were also added. Held cabin cool in 100+ degree heat idling for over an hour. Was stuck at copart lot mid summer in Houston waiting for a vehicle and had 3 people with me.
Edit: Also had the...
I did sanden conversion on my 91 square using stock lines. Had an adapter to extend one of them.
https://www.gmsquarebody.com/threads/r4-to-sanden-whith-stock-ac-lines-for-serpantine.22437/
Often could be a failure of tcc apply circuit or if vss giving bad information. Ecu will see slip with tcc applied and keep increasing pressure, then it will keep 4th completely off until cycle restarts again
Also if there coolant showing up above the front passenger side intake bolts, it can be the heater hose seal. Oring in there can leak at higher rpm while sealing at idle.
Idle Air control set up with the throttle body would need to be sorted, but there are a number of aftermarket intakes with injectors per runner. A lot of them are rectangular port, but still work well on oval port.