HC ohc running gear will fit under an HB....all of it !!, cos that what you will have to swap.
HC Xmember will fit , but you will have to fit an HB rack to mate up with the existing steering col or modify the column...easier to swap the rack !.Right , HB rack has three fixing bolts whereas the HC has two so if so inclined drill extra hole in Xmember whilst its all in bits !
Petrol pipe will have to be rerouted to the drivers side but then you were going to renew it anyway werent you !!.This needs to be 1/4" as opposed to the original 3/16" if my memory is correct.Gear lever hole will need adjusting!...ref back to hc tunnel.
Rear axle will be need to be HC ohc, will fit a treat but remember to swap shorter diagonal arms for ohc type ( even shorter). HC ohc prop shaft will fit without tears, early one piece or later split type .
Now then the HC axle is a little wider than the HB but will cause no probs...does help the wheels fill out those arches!.
Handbrake mech is a little different ...just nick bits off your donor HC!
Not quite a weekends work ...but Ive seen it done in two !!
Hope this makes it a little clearer!.
Cheers Tony