78t spur 29t pinion lands your FDR at 7.0. You may be able to make 78t 28t pinion work however I would use a Robinson pinion if you decide to purchase the 78 Robindon Spur.
80t/29
Recomended FDR per most 13.5 motors used in 10th scale wheelers is 7.0
To figure FDR take the spur divide by pinion, take that multiply by internal gear ratio being 2.6.