340 oil pressure? Help

IF the engine only ran for 5-10 seconds and 40psi is all you saw then what you are seeing is perfectly normal. IF the engine ran for 2 minutes or more and 40psi is all you saw then that is low. Find out why. Should be 70-80 psi cold with a fresh build, don't care if it's a std volume or high volume. Use a std volume until you learn where and when a high volume is applicable. It took me a long time to figure out where and when to use a high volume pump--it probably is not the best knee-jerk solution. I use a high volume about 10% of the time. If you truly only have 40 psi cold @ 2000rpm then Houston we have a problem and you need to go deeper than an oil pump. ----Unless the shop is so shady they installed a VERY worn out oil pump-which is VERY unlikely. J.Rob
p.s. My bet is the roller lifter bodies are undersize or the "band" cutout is in the wrong position causing massive hemorrghing.