The Cummins swap would be pretty straight forward. The addition of the Allison might not be so simple unless you find a tranny out of a larger truck, but they tend to be computer controlled. Making a GM Allison work might prove to be costly? Everybody wants an auto? I get on the highway in Nashville and can drive to Cleveland before I shift gears to pull over for fuel, almost like an auto. Gets better fuel mileage than an auto, less maintenance costs than an auto(oil changes, flushing the tranny) less expensive to repair than an auto(mine has 145k, original clutch you auto guys are changin oil $$$ Got a saturn with 300k original clutch, never touched manual trans) But, I guess some guys just don't like rowing the gears. LoL!
As far as weight goes you would have to find out(owners manual) what it is rated at, GVWR, rear axle ratings for the specific year. Found something on the web for 2005 F350 said 13K GVWR, if that's what yours is you'd be ok more than likely.
Here's a site that I've been going to for years. There is a guy on there that did an 03' I believe with a 24 valve, think it was one of the newer one's with the CP3 on it. Anyhow, turned out pretty slick, saw him run on the dyno.
http://www.dieseltruckresource.com/dev/cummins-conversions-f157.htmlThis site is real good to,
http://www.cumminsforum.com/on that one to actually. Both have lots of information....kinda like here but for Cummins nuts.