Vans owned:
1986 Safari 2.5L 4 speed manual - scrapped
1995 Astro 2WD conversion 4.11 posi, shift kit, DHC rock rails - sold to Skippy
1998 Astro 4x4 D44, D60, NP231, full hydraulic system with 9k# Milemarker winch and snow plow - sold to Lockdoc
2003 Astro AWD all stock - traded for a 3/4 ton truck
2005 AWD, 4.10's - sold to skippy
You will want the flexplate, and as many of the original 1996 bolts (like trans to engine, front accessories, etc, as some 1999 vans are all metric, as well as 2000 and on are reported to be by Hollander.
The problem, they are not all metric, and there is no rhyme or reason to it. When I did my 1999 van, all of the fasteners switched orer from the 1999 4.3 to the 1999 5.7 with no real problems, but I could have just gotten lucky. Mine had almost all metric hardware (wrench sizes, etc), but many if not all of the tapped holes in the block were SAE, like 3/8"x16.
There is something different about the flexplate, offset maybe, I really didn't look that close, I just put a new one on the 5.7 to be safe.
You may check your 99 out to see if it is metric or not, looking at the build date may or may not help, earlier would be SAE and later metric is my guess.