What year did all the chevy blocks become a 1 piece rear seal, and can you change a 1 piece to a 2 piece?

1986 & up were onepiece. There is a nasty rumor that some 1985s were one-piece also.

Oddly, there is no easy method which I know of to fit a later style crank into a block designed for a a two-piece main seal, but there are adapters to fit and older style (bat weight) crank into a block machined for a one-piece RMS: http://www.gmpartsdirect.com/performance_parts/store/catalog/Product.jhtmlPRODID=892&CATID=883.html

1986 & up were onepiece. There is a nasty rumor that some 1985s were one-piece also.

Oddly, there is no easy method which I know of to fit a later style crank into a block designed for a a two-piece main seal, but there are adapters to fit and older style (bat weight) crank into a block machined for a one-piece RMS: http://www.gmpartsdirect.com/performance_parts/store/catalog/Product.jhtmlPRODID=892&CATID=883.html

Exactly as for the one-to-two-piece conversion - I've seen these used.

My block is a two-piece block and it's a very late 1985 model (July 1985) - it does have the three bosses in the lifter valley, drilled and tapped, for the roller lifter retainer, as well as the machined face, bolt holes drilled and tapped for the camshaft retainer. Still a two-piece block, however.