Just to clear up a small error in your posting. The G3 was a Gartrac built conversion (a rally prepared version of the later road going G6). The Genesis conversion was originally marketed by Rally Design if I remember correctly and was based on a kit supplied in kit form or complete shell. It used fresh steel for the floor, 4 link boxes, etc and was designed to take all Mk2 Escort group 4 parts including steering column. It sported standard width arches as standard and I believe a MK2 escort dash panel was specified on complete shells!?
The Gartrac G3 used many specially built/adapted components as supplied by Gartrac and used a Mk3 dash panel and modified steering column.

As an addition, sierra floor/inner wings work beautifully on the MK3 and 4 Escort. Steering column, heater, etc, etc, can be engineered very well. Although this has universally been adapted by many companies and extended to many other models, Gartrac have carried out this conversion to customers own shells. Talk to them. (Not cheap but the best)
Cheers.