3.36 gears were standard for the HO. 3.55 could be had as performance axle option or 3.55 posi. the 3.36 had the FN wide ratio Muncie. 3.90 and 4.33 gears only available with FX close ratio AND Saf-T-Track posi rear.

As far as an opinion...3.36 I have...as well as 3.55...love them both. I get rubber all 4 gears with both and both have M-20's. If you eventually plan on an M-21 AND 3.90 gears...I would say build it right and once. Jim Mino would say...hell with it...I'll build 8 rears. I would stay away from anything higher than a 3.55 with an M-20 and would only use an M-21 with 3.73 and up.

As far as what gear, ask yourself what kind of driving you will be doing most of the time. If a weekend toy, no long trips. go with the 3.90 and M-21 as you plan based on how your car was ordered. If you want a long cruiser, M-20 with 2.78 to 3.08 (different carrier) If a mix, 3.23 to 3.36. If I remember correctly, 3.36 and up, same carrier, 3.32 and down different carrier.

The other issue is the 250 hp with a high gear..it will work, sure, but a little extreme.

I would keep the 3.08 with the 250 hp engine for now. Build the engine you want, keeping the 3.08 and see how you like it, since you already have it..If you like it, add a posi carrier. I do not believe 3.08 was available with an M-20 though...if you don't like it..build a 3.36 or 3.55 rear.

I would go with Richmond gears and an Eaton posi..you can find complete rear swap packages including the races, pinion seal, bearings, gear and posi for around $725