Timing chain.

You paid him to rebuild the motor so it would run, so yes he should fix it. I consider a rebuild to include a complete tear down, block and heads checked for tolerances, machine shop work if needed (and at 300K miles, it's going to need it), then all new rings, bearings, seals, and gaskets. Heads should be rebuilt, new cam and timing chain. It's possible all he did was slap new gaskets on it and didn't touch the bottom end of the motor, in which case it's not a full rebuild. This is the sort of thing that has to be discussed and worked out as the motor is disassembled, you can't always tell how far a rebuild should go until after you start tearing it down. So without knowing what arrangement you had with your mechanic, all we can suggest is talk it over, either it's on him or you didn't pay for a full rebuild and will now have to foot the bill for the timing chain and components.

Thanks alot. That was exactly what I was thinking.