You will need to get a fitting with a 1/8" BSPT male side to screw into the block and a 1/8" NPT female side for the gauge to screw into. In case you are unaware, BSPT and NPT are different thread pitches. Hydrolic stores (look the the yellow pages) will be able to get you the proper fitting. Most only serve industrial customers and they won't be happy when you try to buy one unit from them, but you can call around and see who will sell to you.
I went to Home Depot and picked up a 2" peice of 1/8" copper pipe that I screwed into the BSPT/NPT fitting so that I could extend the sender out. While at Home Depot I also picked up a 1/8" female to female fitting to screw into the 2" pipe. The sender then screws into that fitting. You will notice that the stock sender extends out to clear the block. The Autometer sender will not clear the block, so you need to extend it out with the 2" pipe.
I also tried using a "T" female to female 3 way fitting at the end of the extension so that I could plug in both the stock and aftermarket oil pressure gauges. There wasn't enough room so I only used the Autometer gauge. It might be possible to fit both, but you would have to take further steps.
Another alternative to his process of using the fittings is to buy a special oil filter pedistal from a aftermarket performance parts supplier for RX-7s. Give that it only requires a few basic parts, I had trouble justifying the price and waiting for shipping.
Caution: Apparently water/oil temp gauges are a different case and require a different procedure for them to be accurate.