I fitted a 2.3 essex and 4spd gearbox in a HB once, fitted great, needed a slight mod to the gearbox crossmemeber (turning round and a new piece of metal welded in to take the ford mounting), i already had the propshaft from fitting a x-flow in the HB. The 2.3 v6 slotted in easily, even cleared the bulkhead, i don't know if the larger essex is taller, but even so, the HC engine bay is longer than the HB isn't it, by about an inch or so? The only problem i had was that the HB steering column went straight through the place where the essex oil filter should be, so i fitted a remote filter housing, which just about cleared. The 2.3 essex wasn't great on the power front but very smooth and useable. The 3 litre will be good though
all in all it's quite an easy swap.