The 90" would look very good. Correct scale on mine (3/4) would be 99". The 90 inch prop looks just fine even though it's a tad small.A 60% scaled P-47 propeller would be just under 95” in diameter. I think that he is looking at the 90” but he is focused on proper scale, so he may go for an exact scale.
The problem with the T-51, especially on earlier versions, is the tail wheel assembly is too short. This reduces rudder ground clearance. Later versions have fixed this issue.Generally speaking, do the Titans and the other P-51 scaled replicas sit at a higher angle when on the ground? Seems that way when I look at photos but I can't be sure.
If so, is it for prop clearance?