I have a 1978 351m in an f150 that I made a swamp buggy out of. Truck runs well on pavement but when you get it in the sand it doesn't have any power. Ran a compression test on one side, passenger side I had, 150, 145, 155, 160 dry then wet 160, 150, 165, 160. Driver side 120, 120, 140, 140 dry then wet 120, 150, 150, 145. The plugs are clean, no oil, look like new. I thought if I had bad rings I would see sludge on them? Any ideas? Thanks for your help!