The K-jet head has a different fuel injection setup
than the D-jet head. I think you'd have a hard(er) time
using the D-jet injectors in the K-jet head.

In your position, I would use the K-jet head and the
73 block. I would also take the K-jet head in for a
valve job and I would replace the timing gear or at
the very least take which ever timing gear looks best.
I wouldn't bother with shifting cams around since that
opens things up for some trouble. You'd be running a
D grind cam in a motor otherwise setup for a (I think)
K grind cam but everything should work okay even so.
Better to keep the cam, cam bearings, lifters and
pushrods all together than to put the correct cam in
and worry about mixing and matching valve train parts.

This way you have a solid bottom end and keep
everything else (fuel injection and such) unchanged.

Of course, what I would *really* do is park the 73
somewhere and drive the 74 until it blows up. What
sort of oil does it have? Does it have enough? How
are the plugs? Have you checked to make sure the
gauge is correct? Have you found metal in the oil?
It's possible that the 74 is running fine and the
gauge is futzed up. It could also be that the oil
pump o-rings are incorrectly installed.

I am pretty sure that a B20 can run for years with
low oil pressure...