The oil pressure sounds good to me (especially for a Pontiac) and the leaky rear seal is a chronic problem with them. Rope seals have to be 100% perfect or they will leak. You have to be careful putting the neoprene replacements in because they are more for racing engines that the builder wants to reduce drag. Rope seals were used in engines mainly because the seal housing areas were not precision machined and thus the rope seal would "fill in the gaps".
As far as blowing your dipstick out, that really is nothing more than excessive crankcase pressure that comes from cylinder pressure blowing past the rings. It may stop after you have some miles on the engine. Generally, the rings will finish the hone job that the machinist could not do and this should subside over time. The better the cylinder finish that can be put on a cylinder the quicker everything will seat.