It could also be drag in the gear, causing the nose over tendency. You said it flew well in the air. With warbirds, you don''t want to be able to stall or snap, so having a forward CG is a good thing. You can pull full up elevator and nothing bad will happen. But you do want to be able to get maximum lift. Try a power off stall in level flight, gradually pulling the elevator back. If it stalls, then you have enough elevator throw. If not, you can move the CG back a little or increase the up elevator travel, if possible.