G'day TommyK, There are quite a few differences between the chassis's 2A 109, S3 109, mainly in the position of the cross members under the flywheel housing and the transmission, from memory they will need to be cut off and moved forward to suit the 4 cyl engine.
What you need now is the wksp manual (4cyl) for the dimensions etc of the 4 cyl chassis, and a copy of the chassis diagram for the Series 3 109, all the measurements are in metric so it will be easy to spot the differences, if you lived over here (East Coast) I could have sold you a replacement chassis to suit 4 cyl 109 ex-mil ( I also have a 6cyl 109 ex-mil as well, I was going down the same track with 'Gladys') anyway all the best with it cheers for now Dennis