It appears that it has the correct harmonic balancer for a 350. I know from driving the truck at about 40 mph was a real event, tons of vibes. The rear driveshaft was completely shot, I'm suprised it survived the trip home, but I suspect that was where the vibrations came from. The engine sounds great and had good power, so I don't think the bearings have toasted... Of course none of the gauges are hooked up either so who knows if it even has oil pressure.
I know for a fact this truck was driven quite a bit, I even rode around in it before I owned it. I'm pretty sure it's a 350 with the wrong flexplate now.
Thanks for everyone's input, it's greatly appreciated.