I’ve had this problem with my SLC numerous times. Couple things to help future builders. You don’t need to drain to use the vacuum method, it’ll still work. Two, most air will be caught in the heads. Easiest is to loosen the crossovers on the heads a couple of times to bleed the air. Three, you absolutely have to rev the hell out of them to get the coolant past the thermostat, drilling a few holes in the stat helps as well. Also, Dex-cool is garbage, you’ve probably read a few things about that.