blue van man

ive fitted an engine/gearbox from a sierra 2.8 xr4x4 into my rearwheal drive transit, allso fitted f.w.d transit stub axles, sorted the front shafts ect all up and running, didnt take into acount the diff ratios, the sierra is 3.62 on the front and the transit is 5.13 at the rear the nearest transit ratio i can find is 3.73, is there any way to run the differant ratios?? ie tyre/wheal sizes? and if i leave the 5.13 at the rear what damage will it cause? the transit is a single wheal 51 reg