IF you have a 99 with a 350 your engine is a L31 350 Vortec engine.
You can buy a NEW CRATE ENGINE from GM with 100,000 mile 3 year warranty for $1800.00 or LESS depending upon how good a relationship you have with your GM parts guy or through Summit/Jegs.
Why would you rebuild your original engine when a complete new Long Block assembly engine can be had for that price?
You can also buy a reman transmission for that same van from GM with the same 3 year 100,000 mile warranty for about $1200.
GM warranty service at any GM dealer in North America.
Add labor cost for install and all the other stuff such as new belts, hoses, radiator etc that you were going to anyway and you will have nice running bus with a great warranty on the major drivetrain components.
No brainer IF you are going to keep the bus longterm!
Remember "Drive Fast, Turn Heads, Break Hearts"