could be a number of fings... some crud blocking the oil ways, pressure relief valve incorrectly set, incorrect pump fitted, not enough oil. its not unknown for engine builders to block oil ways by using too much gasket sealant
on some engines you can use an old distbutor drive and a drill to prime the oil system before attempting to start the engine.
if yer lucky someone with more knowledge of the v8 will come along and tell wots up.