I would say fix the frame instead of swapping it out, but thats just my opinion. repair parts for the frame are readily available, and even if you need to take it to a welder to have it done its still a ton of savings compared to the price you quoted. 350 is a great price for a frame but 1500-2000 to swap it out? To me thats just an insane price.
There is 11 body mount bolts, 2 ground straps, and the fuel lines and radiator, and the body is off. Thats about an hr with the proper tools. The drive train takes a bit longer, (maybe a day) to swap from 1 to the other.
So 1 hr to dismantle, 8 hrs to swap drivetrain, 1 hr to re-install body. 10 hrs total for 2000? 200/hr? wish i could get that.