As was said, this is not good.
Is that the same amount of time for the engine to build oil pressure? Did the compression ratio increase with the head change so your putting more pressure on tired connecting rod bearings.
If it was a loose piston, too much clearance to cylinder, they usually take a little time to heat and expand, maybe rattle for 5-10 seconds. Also their rattle is typically NOT a loud knock. Valve train noise is not a loud knock either.
Put a timing light on the engine, then start it up. Knocking that matches the flashes of the timing light means the knock is matching spark plug ignition rate of fire. This would typically mean a rod bearing as the burning fuel is pushing down hard on the piston with each spark.
Without being there to hear it, my guess would be a loose rod bearing, meaning the bearing is worn alot. How many miles on the short block? Doesn't matter to some extent, a poor rebuild could knock after a few thousand miles.
Its a bit of a hassle to remove the oil pan without pulling the engine. I've done it on many cars, so probably can be done. Lift up the engine with a hoist or screw jack on 4x4 laid across the fenders with chain over top of jack and down around the crank pulleys, drop the idler arm from the k-frame, maybe have to disconnect a tie rod end, pull the pan, and inspect every rod bearing. If you catch it early enough could just drop in a fresh set of rod bearings (all 8 rods) and be good to go. Might pull one main cap off just to see how they look, although they usually will outlast the rod bearings. Best to at least plastic gage the clearance between new rod bearing and crank journals. sorry to ramble.
