I think some commers came with overdrives. (some parts of hte system are on the speedy spares commer price list) but certainly not that many. The other thing is the standard Rootes 1725 overdrive gearbox can be fitted (in its various guises). The "car" version has a longer bellhousing and overdrive adaptor, so the mount needs to be modified a fair bit. There are quite a few other considerations, like the speedo cable length and the prop-shaft length, but its all do-able. They are available too, I think some Rootes parts suppliers have them in stock.
I'm not an expert, but I'm trying!
Rupert
www.littleyellowcar.co.uk