Dumb question but I did a search LOL

[87chevy.com]

i don't think that's how the cooling system works on that water pump, plug it and simply use the factory radiator overflow nipple to connect to the over flow tank. that line is more for bleeding the air out of the system through circulation. just don't cap off the heater core circuit, it needs to flow

if you connected that to your overflow tank, it would fill up and spill out... it would be like connecting your upper radiation hose to it...

as far as a/c goes, they sell can bus modules, i use them on my builds to turn the compressor off to simulate factory.
Off with high a/c pressure (330 psi), +60% throttle and rpm 4k+. vintage air only can do a pedal switch... with the l83, you will see full throttle way more than you think.. would recommend looking into setting that up as well.
clint