My 350 overheated and warped the heads, I had them repaired at the machine shop, I replaced the water pump and the oil pump and put in new rings. I did not pull the crank, and I used the original Rod bearings ( being careful to return them to their original location.) The problem is, the engine cranks and runs great for about 20 seconds and then begins to lug down and die, the temperture gauge goes up a little but the lower part of the moter is extremely hot, it then has to cool down some before it will start again. I have about 65psi oil pressure.

 

should have checked/replaced rod and mains. possibly spun bearing. low oil level. incorrect torque on rods maybe.

 

you should have replaced the bearings they tend to fail if an engine gets hot enough to warp the heads indications are bluing of the crank and scoring, bearings are so cheap they should have been replaced on this partial rebuild