Does anyone know when the last year was you could buy an L88 crate engine at the dealership?

Well, its really stretching my memory but I think there was still some availability into the very early '80's.

In my GM performance parts book (Power and Performance) dated May 1989 there is an L88 partial engine assembly (long block without heads). It included a 4-bolt block with provisions for external oil cooler fittings, a forged 1053 cross-drilled crank (#3967811), L88 cam, timing set, forged 4340 steel connecting rods with the best 7/16" boron rod bolts, and forged aluminum open chamber pistons with floating wrist pins. Heads were not included, but you could purchase open chamber cast iron heads (#6260482) or aluminum (#14011076) that had 118cc chambers and used 2.19" intake and 1.88" exhaust valves. The cam had .540" intake lift and .560" exhaust lift. Compression ratio was 12:5:1.

I'll have to dig up my later catalog to see if it was still in the book.