rebuilding a slant 6?

Do a compression test first, if bad, readjust valves and test again. If still bad you will need at least a valve job. If the engine doesn't use oil, I would just do that and save my money for a complete overhaul when fully funded. Figure a budget of $1500 if you're going to do a stock rebuild yourself. If you're planning to warm it up a bit, do some research on the parts you want and add that and another 25% to your machine costs. With your drivetrain, drum brakes, 903 3 speed, and a 7 1/4 rear, I would stick with stock unless you plan on upgrading those too.