A rebuilt engine is only as good as the machinist/mechanic doing the work. I'd go with a used engine (with good compression readings and minimal sludge) and shy away from auto part store rebuilds. There are just too many unknowns with rebuilders. Auto part stores usually go with a lowest bid type builder and the engines are hastily put together. Something as simple as some lint from a red shop towel can foul the works and cause a catastrophic breakdown. Although they give you a warranty, they're just playing the odds at your risk. They might warranty a bad engine, but it doesn't change the fact that you still have to deal with another breakdown crisis and do everything all over again. Jasper is a big outfit. I know they have an apparatus to run check their engines before shipping to check power balance, oil pressure, temps, leaks, etc. That's a huge reassurance and quality control bonus. Smaller rebuilders just don't have the equipment or ability to do this. Fortunately, your 318 is a real durable and straight-forward engine to rebuild.