Hi Pete, welcome to the site! I thought the 6400 series all had a 2 speed rear, but, I suppose they could be ordered either way! My 58 viking 6400 has a 327 in it, (and SM420) and a 2 speed rear end. I towed my 35' vintage travel trailer from Seattle, Wa. to Tulsa Ok. and back last fall and even with the 327 in it, it struggles going up the hills. I do run a gallon of diesel in the tank (1 gal in the stock 17ish gal tank) and it has more power than straight gas (trick thanks to Jerry on this forum, a.k.a. Hotrod Lincoln)
My 327 does not have quite the power a 350 will, especially if you build the 350 and not just use a 500 $ engine that was laying around in someone's shop!
That said, my engine compartment is different than yours so, I can't answer about steering fitment and stuff, but I am sure others will chime in soon with more information.
As far as bellhousing, you need a cast iron one from a truck with a V-8 in it anywhere up to 1967. They switched to the SM465 in 1968 and that has the larger bearing retainer so, larger hole in bellhousing. clutch etc from the V-8 will work fine with the SM420.
Most of the time, the 2 speed rear is slow and slower so, verify gear ratios! You may look for an over/under aux trans off a larger truck to adapt in there.
