slant 6 problem

I agree with what has already been said.

Since the car is new to you I would start with the basics. You can assume nothing until you verify it. Buying an old car is like buying a box of chocolates, right Forrest?

Do a compression check and write down the numbers. That will tell you the general condition of the rings, valves, head gasket, etc. Remove all the spark plugs and prop the throttle plate open with a screwdriver down the carb throat. The numbers should ideally be within 10% of each other. If you don't have a gauge you can borrow one from one of the larger auto parts store chains. 2 adjacent cylinders with very low numbers is most likely a blown head gasket.

While you are at it rent a cooling system pressure tester. Good for checking if in fact there is a leak in the system. You can also check the rad cap with the same tool.

Look at the condition of the rotor and distributor cap.

Check for timing chain stretch.

Adjust the valves.

Check the timing correctly with a light and set it for 8-10 degrees BTDC.

Collect all the data as you go along and write it down.