I don't have experience with the gassers but do have a couple comments for you:
1) The 3.73 should be all you need with the 6 spd tranny Ford is using now, I would think 4.30s would kill you MPG.
2). Unless your hanging a snow plow off the front, with a gas engine I would think the Heavy service front spring package would make the ride awful rough.
Tim in Central Florida
2013 E350 Cutaway Dually gasser w/10' Knapheid KUV Toolbox
2012 F350 CC SB Lariat Tuxedo Black w/RollnLock bed cover
2012 F350 CC Cab/Chassis with 10' Knapheid tool box
2006 F250 CC Lariat with ARE construction cap
2007 F350 CC Lariat with 8' Knapheid tool box
2005 F350 CC King Ranch with ARE construction ca
And, sorry to mention on this forum
Two 2012 Nissan NV3500 High Roof cargo vans