Before you invest in an offset wrench, try loosening the nut with a 15/16 socket (am assuming you already have one in your toolbox). The pros use sockets with an impact wrench.

Before loosening anything else, put a socket over the nut. To extend the reach of the socket try 1/2" washers between the 1/2" wrench and the socket. Tap the wrench sharply to mimic an impact gun and/or extend the wrench handle with a pipe over it (thus creating a breaker bar).

(This is just to loosen the nut. Don't remove the nut completely until you install the spring compressors.)

It that works then tighten nut last on reassembly. Maybe even place a couple of 1/2" washers under nut so nut is easier to remove with socket next time.
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1980 245 Canadian B21A with SU carb but electronic ignition and M46 trans in Brampton, Ont.