I have a sb400 40 over 6" rods, pre 66 327 heads ported victor jr Holly 950 HP carb.
maybe 400 to 500 mi. on rebuild.
It ran hot one time, thermostat may have stuck, don't know but started seeing bubbles in over flow. No water, oil mix that I could see. so I had heads serviced and they are fine. I replaced gaskets, it runs ok just like it did from day one, Now has about 200 mi on it and now I just check averything been ok till I seen oil in cooling system. It runs cool at 185, oil is at 60psi

 

When I had that problem it was a bad intake gasket. But if you just had the heads serviced and replaced the gaskets it should be ok. Did you replace the intake gaskets as well?