i think i may have found a solution for me.
i found this table showing ratios


if these figure are correct.
now i have a mk4 escort cab which shows the same final drive as the s1. so im thinking if i drop the box out of the 3i strip the output shaft and install that in the s1 box i can fit the rs1800 ib5 or the s2 box in the 3i.

What do you guys recon none of them move more than a few miles per year there just ornaments realy, would also buy me some time to find some boxes/shafts