Best IRS donor gearbox?

[carl4x4]

I'm having my Ghia converted to IRS and I have been offered the choice of a few gearboxes. The car will be running a 1955cc long stroke engine with hopefully plenty of torque. The aim is to use it as a Sunday car as well as having some fun hill climbing and sprinting it on track a few times a year.
I'm wondering what the best donor box is

I've been offered a 1303 box, a 1302s AH box, and a 1303s AT box
I believe the 1303 box has a 4.375 final drive
the AH 1302s a 4.125 final drive
the AT 1303s a 3.875 final drive

First question is my research right?
Second question, which box is going to be most suitable for my combination of driving? thanks in advance for any feedback

[FJCamper]

Hi Carl,

The 3.87 is for top end. The 4.12 is the best all around for accelleration and a strong midrange.

I had a 3.87 in my street Ghia. I could hit 90mph in 3rd.

Some posters might have ideas about single and dual sideplate transaxles. I'm commenting on the gear ratios.

FJC

[Bruce2]

For hill-climbing and other racing, there's no question. The 1303 'box is the way to go. But don't expect to be able to cruise comfortably above about 65mph or 100 km/h. If that is important, the AH is better geared. But, the AH is most likely a double side cover gearbox. Not as strong as the SSC of the '03 or the '03S gearbox.

[Fiatdude]

002 bus

[carl4x4]

Hi Carl,

The 3.87 is for top end. The 4.12 is the best all around for accelleration and a strong midrange.

I had a 3.87 in my street Ghia. I could hit 90mph in 3rd.

Some posters might have ideas about single and dual sideplate transaxles. I'm commenting on the gear ratios.

FJC

Thanks for the feedback everyone, I've decided on the 1302s AH gearbox for a happy medium, nice to here some people with experience confirming my decision
Regarding the gearbox strength I'll have it rebuilt with a superdiff plus a few other bits to strengthen it up, but I want to run the car for a while with it standard to make sure I'm happy with the ratios before I do this.

[Marc]

The First gear in the AT is far stronger than that of the AH...better IMO to use the AT and 6% shorter tires.