Anti-freeze losses . . . . . .

[Ingoldx1]

89' 4x4 sub, rebuilt 350 with headers and duals and a t350 tranny. New heater core, intake manifold gaskets. No visible leaks anyware. I have checked all of the hoses and connections, no luck. I have found that the motor was burning the anti-freeze. I found some on the drivers side plugs when the headers went in. The exhuast manifolds had white areas on both sides (d/p side). I did a leak down test today (engine warm) and had a drop from 17 psi to 13 psi in about 1h 45m. I have the rear heat option and the control valve that goes with it if it helps. A weak before doing the leak down test I ran a bottle of Bars stop leak in it. I think the heads are bad. I have never had any anti-freeze in the oil. I have not any problem with the temp. The gage does not jump around or run high.
Any one with any experence with this?
- Ingoldx1