Correct me if I wrong, but from what I've seen some other people do is leave all the firewall solenoid wiring the same and put a jumper wire on the starter's solenoid between the Bat and Ign terminals.
Yeah saw some others try that method, but some had cranking issues. Didn't want to take my chances, so I followed the way that Tuff Stuff recommended. Been a couple of years and haven't had a single issue cranking.
Could you not have just continued to use the starter relay on the firewall and connected the hot from that relay to the output side of the relay on the starter? Then you could have just used the existing cable coming from the firewall relay and the original key switch wire.
Thing is, since now there are two starter relays (the new starter has it's own relay, while the old one didn't), if I were to hook up the hot from the relay on the fender apron directly to the starter, while I cranked the engine over, it would send voltage back to the relay, keeping it energized. It would continue to crank even after I stopped turning the key. So the starter relay on the new starter needs to be triggered separately. Saw some folks in the forums run it the old way and have that cranking issue, so figured I might as well follow the instructions and hook it up as they suggested. Been over a year and haven't had a single issue with it so far.