Need help here for a person without a lifetime of experience with GM motors. Looking at some used racing parts and need explained the differences between the two piece seal type pan and the one piece seal type pan. I'm looking at an eight qt. pan with the dip stick on the passenger side. It's my understanding that makes it a later ( newer ) model pan ? Any differences ?

 

Depends on your block. 86 and later are 1 piece rear main seal. Earlier were 2 piece. The rear main seal design changes your oil pan requirements.
The drivers side dipstick would have been an even earlier block with a 2 piece rear main but before they moved the dipsticks to the "safer" side of the vehicle.

 

That confirms what I thought about what side the dipstick was on. But what do I look at on the pan to tell the difference ? I have a '91 block

Thanks

 

91 block means it's a 1pc rear main. That's all there is to it. Get an oil pan for that.