The rear pump is the expensive one I have a 88 930 and I just replaced the rear pump for an alternate pump.
First download this pdf http://apps.bosch.com.au/motorsport/downloads/fuelpumps.pdf
it is the full specifications for Bosch motorsport fuel pumps.
My 1988 930 has a front pump with the number
0 580 254 967
and a rear pump 0 580 254 979
Your 1976 930 probably has the same setup. (For info the rear pump was also used on 928's and 944 turbo's I believe)
If you look at the motor sport pdf you downloaded you can see that there are two alternative pumps with the same specifications. The only difference is the inlet connection.
I purchased a 0 580 254 984 for $200 cheaper than the original and fabricated an adapter (Actually just a piece of 3/8" fuel hose clamped over the fitting).
I am sure that there are other alternatives out there such as pierburg, ACDelco or walbro. But I couldn't match the fittings/electical connections or mounting easily.