Puk'in Fluid

Just finished up assembly of original 69' 340ci +.040 10.5:1 and running on run-in rack. X-heads, Edelbrock Performer w/650cfm spread bore and 292/.508 hyd cam. Sounds great and runs for 10min then boils over and pukes about a gallon of water out. Heat cycled twice and re-torqued heads. Edelbrock top end gaskets used and assembled correctly. Removed thermostat with same results. Using original 69' style iron water pump with full flow bypass hose. Thinking of changing to late model alum water pump but lower hose is on opposite side. Ready to install in 65 valiant 200 coupe but stumped on this one. Any experiences are appreciated. I'm done with guessing.