I had a 3.90 behind my T56 and 383 (about 450HP or so) and felt it was fun but too short. I sometimes would just start off in 2nd...(which was fun too!)...Anyways my new rear has a 3.73.
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1969 Camaro/ Tom Nelson TT 434 / Wayne Due C5 / DSE QLink / and a bunch of other stuff...
So my choice of 3:70 (9 inch Ford) in my Monte/LT1/T56 was good? I was concerned with a short first on the street, and useless 6th on anything but wide open highway.
I'm running a D&D T56 and 4.11's in my Fab 9. First gear is fine,definitely not too short. The 3.50 cogs I was running didn't allow for a very useable 6th.