Gas In Crankase

HI,
I have a 1975 454 GM Big Block V8 7.4 Litre engine. The problem is I am getting gas in the crankcase. I have replaced the Holley four barrel carboratour with an Ebelbrock 750CFM, as well as the fuel pump, the fuel lines from pump to carb, air filter, the thermostat (180 degree), new MSD 8.5mm plug wires, new plugs and a new distributor cap.
I took the Ebelbrock Performer 2 intake manifold off and had it checked for cracks. Unfortunately there were no cracks but the manifold was worped. The mechanic machined everything back to spec and he also mentioned that when he removed the thermostat it wasn't seated properly in its groove. Everything is back together now. Yesterday was the 6th time that I changed the oil in the past two weeks and I am still getting gas in the oil.
I am getting 3.5 to 5 psi gas pressure. It is never constant, it always varies each day. The engine runs fine, there is no problem with starting or idling but after 2 hours of constant running there is around a litre of gas or less in the crankcase. Could anyone help with this problem?
Thanks!