Is it getting spark? You might want to make sure your timing is good and your wires and plugs are all good. If its been sitting a good thing to do is change the plugs and wires.
If the 4.2l wasn't stock, and they didn't change the rear and front end, then you'll have to change that. A V8 has too much torque for the gears in them. You might have to change them even if they did change them to something that would work for a Straight Six.