Converting M-21 to M-20 or T-10

[Elm Zobens]

My 69' coupe currently has the 10/27 spline M-21 4-speed with a 3.70 axle.
To get a little more kick off the line with my ZZ3 small block, I would like to swap this trans for either a M-20 or T-10 with a numerically higher 1st gear.
I have a rebuilt M-20 and T-10 but both of these units have the 26/32 spline arrangement used in 71' on up models.
Has anyone done a swap like this before?
I'm guessing I need to swap the clutch disc, the slip yoke and the differential yoke (and possibly the driveshaft) to facilitate this swap?
My other option would be to replace the input and counter gear on the M-21 to a M-20 unit but that would end up costing more money- Even though I wouldn't need to replace the clutch, yokes and driveshaft.

Any comments or advice?
Thanks!
Elm.

[Mark #28455]

do either engine or trans not both

Saw your other post about the engine. If the engine is running well, save your money by leaving it alone for now. Instead, invest in a good 5 or 6 speed trans with the complete installation kit. That way you still get the first gear punch with an overdrive for your 3.70 rear too.

There hasn't been a performance car built with a 4 speed in many years. If it's so great why do all the Vipers, Corvettes, Mustangs, etc all go for the 5 and 6 speeds?

[Mike M.]

Re: do either engine or trans not both

agree with mark. install a 6 speed and throw away the 3.70:1 in favor of a 4.56 or 4.88. that'll get your attention and the milage will improve. mike

[Elm Zobens]

Re: do either engine or trans not both

I appreciate the feedback!
A 5 or 6 speed conversion would be nice, but the budget for such a swap would probably cost twice as much as the trans swap and engine crank swap combined.
I've even toyed with a GearVendors OD unit behind the 4-speed at one time, but again that is an expensive option for what my needs really are.

I don't do a lot of freeway driving with this car, so I really don't mind the 3000 RPM at 60 MPH it pulls currently with the 3.70. I'm not looking for an 11 second street car but 13's would be nice. My 04' GTO does 13's with a 3.46 gear at 3700+ pounds- I would think I could do the same with a 3200 pound Vette and a 3.70 gear considering the engine specs are similar (GTO- 350HP/365Ft/Lbs vs ZZ3- 345HP/380 Ft/Lbs)?
Thanks again!
Elm.