The PRIMARY determining factors are what main and rod bearing clearances your builder used. He would have accounted for the expansion of your ally block in the bearing clearances he selected. Call him and ask those clearances and post back here. .002" to .0024" would be good for the mains. Rods maybe .0023" to .0026".
Then
oil viscosity and temps. 10 psi per 1000 RPM is an accepted standard. 20 to 25 would be nice for a hot, 1000RPM idle for FE's. But 18 won't melt the motor.
Avoid the problem by no prolonged idle less than 1200 or so.