Spicer 20 spd

John K You are correct as I was told the same thing that there was only one overdrive gearbox in the AACO series and they are getting hard to find nowdays. I was lucky in finding a very good one

Thanks for the offer Lang but you don't need to do that because I have got the dimensions of the Commer box and it presents difficulties with the aforementioned chassis cross member. I have now also had a chat with a very helpful Ed Eminson from Dubbo who has helped me work through my confusion. Given my problem with a chassis cross member being in the wrong place I am kinda going back to my original idea of a joey box now. It seems that any gearbox I get is likely to clash with said cross member and I may or may not be able to use the bell housings that I have. On top of that, I already have the highest ratio diff that is available in a bus format so a joey seems to offer easier mounting and better ratios.

I wanted to do the same thing with VP, could not find a 2 speed diff (mounted backwards) to suit, would have fitted a Gear Vendors OD unit, but sadly they don't make big enough.
That was when the 7 speed came up so we took that option. Big improvement. Even one extra gear can make a huge difference.

Kwakatriple
Glad to see you are sorted.

Lang