If it's off an Arrows series car (Hunter) then there are differences in the bell housing and input shaft. The Hunter one is deeper and is made so that the gearbox is at a different angle to earlier cars. Best way to check is to compare the depth of the bell housing to your existing gearbox.
If it is an Arrows box then you will need to use you existing bell housing and the Commer input shaft fitted with the Arrows 1st gear wheel.