Pong left by a ping

The 351 has quite a high compression ratio and may need fuel with a higher octane rating than the 96 of LRP. It may also be in need of a de-coke to clean out carbon build-up in the combustion chamber, which tends to increase the compression ratio. First, of course, check the ignition timing to make sure that's set correctly. As for the future, it really depends on what you plan to do with the car. The ZD is a lovely car, but it's now more than 30 years old and it may not be worth spending any money on it. If you intend to keep it, I would spend the money to have the heads rebuilt with hardened valve seats to cope with unleaded fuel. At the same time, I would lower the compression ratio with new pistons to cope with the lower octane rating of ULP. Do that and you won't have anything to worry about with fuel supplies.