Re: Converting E-150
Depending on emissions in your area, I'd look into that first.
Keep in mind also that the E-150 isn't meant to carry insanely heavy loads. I'm sure the V6 version could carry 2000-2200lbs without much worry. Dropping a larger engine in there will reduce cargo carrying capacity. The 4.2L V6 had great torque for its size and is decent on fuel. The E150's suspension will require uprating front shocks/springs etc to better handle the weight of a larger engine.
Additional upgrades to think about:
Transmission - the one mated to the V6 won't run too long connected to a higher torque V8
PCM - you'll need the computer from the engine you plan to install, including wiring harness
If you are really into upgrading to a V8, I'd suggest a newer 5.4L block as they have great low end torque stock at 350lb ft @ 2500rpm. They can also be easily found at local salvage yards; perhaps with a transmission and PCM!. If I had a V6 2000 van and wanted a V8, I'd go newer and put in a 5.4L block. You can add go fast goodies to this blocks as well, as it's a common build engine in the F150 trucks. Heads, cam, custom tune, free flowing exhaust system should easily allow you to bring a stock 5.4L (250-260hp depending on year), up past 300hp.
I'm not sure how easily a 351 with proper transmission would bolt up in place of the V6, but, again, I believe the 5.4L would be the way to go. If the van was 10+ years older, a 351 would be what I would suggest.
If you can find a great deal on a 351, and get a transmission to match, or an appropriate bell housing to utilize the stock transmission and build it up, go that route. For me, I'd go with price and simplicity. Either build will take time and money, but if you shop carefully, I'm sure you'll find the 5.4L hard to resist.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!