Causes of unsteady idle/WTB heater control valve?

I got two questions here... First, I recently built a 360 Magnum for my '70 Duster which runs GREAT except for a bit of an unsteady idle. I've made sure the initial timing is correct (~10* BTDC) and played around with the idle mixture/speed but it still seems to fluctuate by about 100 RPM and my vacuum gauge goes down by about 0.5-1" every couple seconds; it's not really steady just every 2-3 seconds the engine slows down a teeny bit then speeds back up. I only have about 200 miles on the engine which has a new bottom end, is this normal for a fresh engine to do? Specs are 10.5:1 static comp, Lunati Voodoo 60401 cam, Air-Gap intake, and Carter AFB 625 cfm carb.

Second question, this same car came with A/C from the factory and has a control valve which shuts off flow to the heater core when the heater is turned off. Mine started leaking so I just bypassed it for now but would still like the functionality in warmer weather since it no longer has A/C and the cabin gets pretty hot with the heater core always 'on'. Where can I get a new one of these, they seem kinda hard to find?