There is only one way that engine speed can increase without road speed and that is to either be in neutral or the clutch IS slipping. You don't say if it's a 9 inch or 9 1/2 inch clutch you have. I would pull it apart before you ruin the flywheel, the clutch disc is already toast. Measure the clutch and make sure it's a 9 1/2 inch. I would machine the flywheel and use a new pressure plate and clutch disc. The old Ford 1/2 tons with a V8 used that 9 1/2 inch clutch so it should take the power you are trying to feed to it.
1971 series 2a 88, series 3 trans, Fairey OD, owned since 1978.