Since you're starting with a 90s, you're at an advantage since it's already fuel injected. You'll just need to swap the pump. Older vans have to swap tanks, lines, pumps, etc. The changes you'll need to make will depend on your donor engine.
There's a whole Facebook group called "LS Swapped Chevy & GMC G-Series Vans (1971-1996)". You'll find a ton of info there. Also check out my post in the Readers Ride section that outlines a lot of my swap.