Are you sure you're running an M20 and not an M21 or M22?
I'm running an M20 with a 461, 3.42 gear and 28 inch tire which works well. I'd prefer a little more gear but don't want the extra RPMs at freeway speeds. It's all a compromise.
With an M20, if anything on your combo with a 25" tire I'd think you would be looking for less gear.
The 1st gear multiplication factor on my combo is 3.31 x 2.52 = 8.34
Yours with an M20 would be somewhere around 3.85 x 2.52 = 9.7
Yours with an M21/22 would be somewhere around 3.85 x 2.20 = 8.47. Still higher than mine
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1970 GTO-vert, 461, eheads, stump puller, m20, 3.42
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