ive got a tdi escort van with a non turbo diesel ib5 gearbox fitted, it accelerates fairly quickly but has a top speed of around 85mph which means at motorway speeds its at high revs

I have a spare ib5 diesel box plus a mk5 fiesta 1.25 ib5 box, I had the idea of opening both up and swapping the gears over or just swapping the final drive

anyone got any ideas of the ratios of the 2 gearboxes so I can try and work out what gears will help?