If your really after 250HP and better economy I'd suggest a engine swap to a Vortec 4200 inline 6 out of a trailblazer. The 88 up vans have TBI injection that can donate the EFI fuel tank/fuel lines/in tank pump. The 4L60e will bolt in using factory parts. You should see a 20 to 25% increase in economy over the 250. At $1.30/L it won't take long until your saving money.
You really ought to take a drive down to Summerland and have a look what the other local inliners have done. It may be the best 3 hours you spend on this project. By the way there is an active Inliner's chapter in the Valley.
The 4200 is 275 hp stock without the hassle of the turbo.
http://www.blackopel.com will give you the contact numbers to call.
