In the same process now with my 79. Agree, an extension with a 13mm swivel end works well to remove the old HEs. I have not installed the SSIs yet, so not sure if the C shaped wrench is necessary, maybe not. I had my engine out and upside down, but the important part is not to snap a stud, removing the old HEs. I did use an oxy-acetylene, and found that when they are ready to come loose, they only need about 15-20 lb feet of torque (measured by my hands). If you find any stubborn ones, do not force them. I ended up with no broken studs, with 1 stud coming out of the case, which I later re-installed with red loctite.
The next challenge, is removing the oil lines from the T stat. I failed at this, even though using heat and penetrating lube. I tried to remove the outer line (to the oil tank) first, and those threads came right off with the nut. Then I took extra special care, on the inner line (t stat to engine crossover) and even with extra special care, those threads came off too. I ended up buying a new t stat. (yes I know there are some weld on repair fittings).
The final challenge is the cross over (actually cross under tube). Seems like all the aftermarket ones, including on Pelican, are poorly fitted, and neither clear the engine case nor the SSis.
This is a good thread, including my experience.
https://forums.pelicanparts.com/porsche-911-technical-forum/1004113-ssi-install-derailed-out-spec-oil-lines.html
You may be able to send your tube to Len (page 3) to have him bend you tube to spec. I think I would recommend this, as getting my tube to fit was a PIA, and that was on the engine stand. Though you may well be more handy than I.
Just sharing, since we have same car / same SSIs. I have test fitted mine to be good, at this point, and should be installing them as soon as I get an O2 bung welded on (for EFI conversion).
Also, just saw a video on youtube by Klassic automotive training, where they installed what appeared to be SSIs, and recommended using 12mm nuts, so you can install with a swivel extension. (No c shaped wrench). Maybe something to look into.