Mine was a cracked block, at the top of the cylinder jacket. Hope to run new mill next weekend. I was feeling pretty good about the new build until I started reading your post. You got me scared now

Intake gasket, You'll check that first before you pull the heads.

cracked oil cooler. Had a bad one once. Took me forever to figure it out. Take it to a radiator shop have them pressure check it. Might not be it but its one more possibility.

I've been down this road also.

Cracked block or heads unlikely. The machine shop should have caught that.
If it is running strong, I would say a head gasket is also unlikey.
My vote would be oil cooler first because it is easiest to check, then intake manifold. How much water pressure are you running?

Second that... Especially if you are running a crossover(no water pump). Assuming you are, check the gasket near the back water passage on the intake. Have an aluminum (Brodix) headed stroked motor, 11:1 compression, HVH intake and due to the fact I was running a restrictor in the thermo housing, and just a crossover w/bypass, too much H2O pressure was building and pushing past the intake gaskets at the rear passage. Never saw a rise in oil level, just a nice chocolate shake. Could idle for days without any water getting in, but take it past 4000 and wallah instant shake. Didn't have a pressure guage installed, so I can't give you any numbers. Hope this helps.

Did the same to mine after taking a 502 to a 540. Using Dart Pro One aluminum heads. It was the intake gasket. Put in a Mercury relief valve, changed to the right gasket and no water!

Im in for a intake gasket. Make sure you use the orange silicone around the water prts both side of the gasket and stick it on good. j

Intake gaskets, what's your water pressure? My Eliminator had the same problem, under 4000rpm everything was perfect. Over 4000rpm milkshake in the valve covers and catch can. Pulled the intake and had pudding. Changed gaskets numerous times until I checked the water pressure. put a relief valve and a dump out the transom, problem solved Good Luck, hope it hasn't hurt anything else. I had a bearing failure the next spring from the water.

Run it at idle for an extended period after warming the motor up. The pull the exhaust away and check for signs of water at or near the exhaust ports. If water then bad exhaust.

Check water pressure. Could be pushing gaskets.

Do you have the correct head gaskets? Are you installing the intake with care and not over-applying sealant to the water ports?

Could be a bunch of things here but I would check the obvious first... Exhaust.

i have a relief for steam pockets already.....My water pressure is around 4-5pounds at idle and about 18-20 at wot.......