Years ago, I traded for a '97 extended cab short box 4x4 with a 4.6 auto that had 275,000 miles on it. The truck had been through hell, the kid I got it from was 16 when he got it and drove it for 3 years. He tipped it on its side and all sorts of good stuff like that. Anyhow, the oil pump had gone out so I traded him for it and found an engine in a wrecked truck with 288,000 miles on it. The engine ran great with no smoke, so I bought it and put it in the truck. I drove it for 2 years and put another 40,000 miles on the truck and sold it back to the kid I got it from. Well it passed through a couple other people over a three year stretch, still running strong when it left the area.
So my point is, don't be afraid of finding a replacement with some miles on it. So long as it runs quiet with no smoke I wouldn't hesitate, so long as the price is right. What I said before about my offer on a set of heads still stands, they're complete still on a running engine that spun a rod bearing. I would send the with cams and all so you'd just need new chains and gears.