Well, a 360 crate should be about 380hp, and with some minor tuning and carb adjustment you should be looking at the 400hp mark without trying too hard. You say it's got a "Hemi" 4 speed? As I recall, that would be an 833 trans. certainly a tough gear box, the rear you described sounds like an 8 3/4". There shopuld be a tag on one of the bolts on the rear center section that will tell you the rear gear ratio. If it's not there, you can jack up the rear of the van, make a chalk mark on the driveshaft, and on the right rear tire. Have someone turn the tire one ful revolution, and count the number of times the drive shaft turns. That will give you a rough estimate of the rear gear ratio (for example, if the drive turns about 3 1/4 times to one tire revolution, you've probably got a 3.23:1 gear in there.
With a drive train like that, it would seem that two things are true: 1. It's not factory, someone built it, and 2. you better have a really good clutch!
Oh yeah, one more thing, it should be pretty damn fast already!!!
regarding up grades, I would recommend looking into opening up the exhaust with 2 5/8" or 2 7/8" headers, some FlowMasters, and maybe a 3" dual exhaust. Look into your cam grind, and check to see what size (CFM) carb you've got.
These are all things that will improve your performance without too many band-aides on your wallet.
LET US KNOW SOME OF THE OTHER DETAILS.