Gear questions
 

I have a 69 with a 400 sbc and 3 speed with 3.73s. During the build I swapped in a new posi but kept the factory gears thinking everything would be fine. I know from the factory the different series diffs align with certain gear ratios but do they make aftermarket longer gears i something like a 3.08-3.27 for the series 4? I hate to buy another posi but my first gear is useless and I’m running 3500 rpm’s on the Highway.

Re: Gear questions
 

You can go the opposite way (higher ratio to lower ratio) with a special "thick" ring gear, but you can't go lower to higher without a carrier change.
The pinion gear diameter is much larger on a 3.07 than it is on a 3.73+, therefore the ring gear has to be further away initially.
You could swap in an OD transmission and keep the "fun" gear.....but it would be more involved/expensive.