OK good find but is it off an Arrows series or an older model? If it's the older type then it will bolt straight on to the engine
Things to check are the splines on the input and output shafts, some are fine splines, some are coarse splines or even one of each either end. Not a problem on the input you just use the correct clutch plate but if the out put different to your current prop shaft then you will need to find one that matches - the prop will need to be shortened too as the OD box is longer.
If the other box you've found is a J type then then the housing should fit the one you've got whether it's an Arrows one or not.
J type I fitted to Harvey



This link might be of use during your re-build
http://www.buckeyetriumphs.org/technical/jod/JOD1/JOD1.htm
No Commers aren't over priced - they're just catching up with definitely inflated inferior vehicles
If the other box you've found is a J type then then the housing should fit the one you've got whether it's an Arrows one or not.
J type I fitted to Harvey



This link might be of use during your re-build
http://www.buckeyetriumphs.org/technical/jod/JOD1/JOD1.htm
No Commers aren't over priced - they're just catching up with definitely inflated inferior vehicles