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Overdrive in a Classic Mustang

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Iíve got 3.91 gears in my 428CJ powered í70 Mach 1 w/ a C6 automatic transmission. The car was rebuilt with street/strip duty in mind. I love the performance of the 3.91 gears but I hate the resulting high RPMS when cruising on the interstate. Iíve decided that I need an overdrive solution, and I am looking for anyone with experience in any of these solutions or perhaps some others. The 3 solutions Iím considering include:

(1) Bolt a Gear Vendorís over/under drive unit to my C6. Cost is around $3000 with shipping and requires a shortened drive shaft. This unit supposedly doubles the number of forward gears, allowing you to shift to 1st (C6 in 1st, GV in low), 1st high (C6 in 1st, GV in high), 2nd (C6 in 2nd, GV in low), etc. They donít lower 1st gear like some of the OD transmission options, but they do provide half gears allowing the motor to remain closer to peak RPMs when drag racing. I donít follow how the shifting is accomplished but apparently pushing a button on the shifter allows you to control the GV unit. Apparently they make this process much easier than it sounds. Final drive ratio is 3.05 instead of 3.91 so the motor is turning 22% lower RPMs on the highway. I can keep my converter, existing shifter, speedometer, etc. I understand that the speedometer setup can be modified so that it remains accurate even when the GV unit is in high gear. I may or may not have trouble getting the unit to fit in the tunnel.
(2) Retrofit an OD type transmission using a conversion kit to allow the new transmission to bolt to my 428CJ block. Costs vary. The conversion kits are $600 to $800 and I can find the transmissions from $650 (good used 700R4 a friend has) to about $2200 for a hardened E4OD. The 700R4 has very nice gear ratio (3.06, 1.63, 1.0, 0.7). Its first two gears are lower than any other OD tranmission I found. The Ford 4R70W transmission can be also obtain with a wide gear ratio (2.84, 1.55, 1.0, 0.7). The advantage of using one of these transmissions: lower first gear is 3.07 or 2.84 with a new tranmission, 2.46 using the C6 + GV; a new transmission does not require an additional mechanism to shift the GV (may or may not be an issue); depenidng on the transmission, replacing my transmission can be cheaper than the GV unit; replacing the transmission lowers highway RPMS by 30% compared to 22% for the GV unit (2.74 final drive compared to 3.05); the 700R4 supposedly has lower parasitic horsepower losses and weighs a lot less than a C6+CV unit. The disadvantage is that I may still have to use a button to enable overdrive because I want to retain my stock shifter, it will require more labor - especially if I go with the 700R4, and the result is 4 foward gears while the GV option results in 6 gears.
(3) Buy a PA built Ford-type overdrive transmission that (if I read correctly) has an FE compatible bell housing eliminating the need for the $600 to $800 bell housing adaptor kit. The problem is that the PA transmissions are expensive making this the most expensive option ($3500 to $6500). It also elminates the most attactive transmission option: the GM 700R4.

Has anyone added some type of overdrive (GV or OD tranny), especially in a classic mustang? What was your experience like? Can anyone shed light on how what the driver has to do to shift through all "6" gears when a GV unit is installed? How seemless is it really? How does the GV solution integrate with the console mounted automatic shifter?

Currently, I am looking hardest at either buying the GV unit or installing the 700R4 with an adaptor kit. If I go with the GV kit, then I may also look at the cost of lowering my 1st gear and 1st and 2nd gearin my C6.

I changed from a 4 speed top loader to a TK0600 tremec 5 speed so I dunno about the auto overdrive but I can tell you having an over drive makes the car totally different and its almost worth the money.. Quieter less vibration better gas mileage my wife actually enjoys riding in my car now..
I dont have experience that way with autos.. but I would spend enough to do it right. I did  on mine the conversion was close to 4 grand. Like a totally different car..

Like Jethat I upgraded my 4 speed to a 5 speed and have been very happy.  I went a cheaper route and used a Keisler modifed T-45 that has a low 1st gear, my mechanical clutch and original bellhousing and a deep overdrive.  As has been said, the car is totally different and mine has both a tire shredding personality and a civilized personality.

Of your 3 choices I would be inclined to go with the 3rd option and use a PA AOD built for your power levels.

ol dirty doug:
i had one on myfairmont wagon, 302 with a c4 and 4.56 gears.  i now have a fully built aod with the close ratio gear set and a tall overdrive ratio.   i also have my 1927 roadster chevy 377 turbo 400 trans with the same gearvendors unit just had to get a different adaptor.  love them both but if you have the money build a aod. its less hassle and easier to use.

The only fear with the AOD is my console shifter floor shifter.  It only has 1st, 2nd and Drive.  I would like to keep the floor shifter.  I understand you can control the overdrive with a button just like the GV unit.   But I was told that you have to remember to switch it out of OD or that the transmission will not downshift when you slow down. 

What options are there to use my original 3 speed shifter with a 4 speed OD type automatic transmission?


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