As mentioned, be sure the van is running as well as it can stock first. Plugged cats, dirty aircleaner, dirty injectors, etc can all cause power loss.
The Chevy is running at about 32# per horsepower if at full 9600# so like any vehicle with that ratio it is not going to be a dragster, especially if you get near 10K feet.
For comparison, my old Escort station wagon had only 28# per horsepower and nobody ever confused that vehicle for anything fast moving.
Hondo did have tune done to his Chevy and thought there was more power from it, and I think those posts should be on the forum somewhere.