Yep it's basically the same engine but with a bit more power (alloy head and slightly better cam) If it's from the later Arrows series cars then you will need to swap a few things over from the Commer engine but it's all bolt on stuff - water pump, front engine plate (for engine mounts) sump pan, flywheel - I'm sure there's a few more but easily done. You will also need to fit suitable inlet and exhaust manifolds and sort out an exhaust
The gearbox is a bit more awkward if it's from the Arrows Series (boxy Hunter type) as the engine and box sits in the Hunter bay at an angle and when the engine is placed in the Commer (with the above mods) it sits vertical. This means the Hunter box will need some work for it to fit and work - but that's another story.
I guess the first thing to find out is if the engine is out of an early car and is a 1725, as some of them were 1600 cc, if so then happy days it will go pretty much straight in but if a Hunter type engine and box then expect a bit more work. Is the gearbox you've been offered an overdrive one@