There's some conflicting info here:
Small chevy 350 blocks will have a drivers side dipstick up to 1980. 1980 was when all of the small blocks went to passenger side dipstick. You will find one piece rear main seals were phased in in 1986. So it's true one piece rear main seal blocks will have a passenger side dipstick however so will two piece blocks so don't use that as a guide. You can tell by looking at the flexplate. If it's a counterweighted 350 it will be a one piece. The oil pans are different as well 55-79, 80-85, 86-99 and there are variances within those years too.