the 305 heads will give added compresiion yes, so much so, that to be able to use them, youll have to use dished pistons to run pump gas, + the 305 heads dont flow ...although on a van Im gonna assume thats not a big issue.
I think I use World Products s/r heads for that application...whatever heads you choose...make sure to add steam holes to the heads for compadibility with the 400.