Can we assume this is a new oil pump, provided by the fellow who built the engine? Lots of 273/318/360 parts interchange, but I think that there is something about the rear mains being smaller on the 360 than the 318 that prevents pans from swapping? Are you 100% sure that you have the right pump for your 318? Have you gone back to the "engine fellow" and asked about the interference? Simple questions, but something just doesn't seem right here.