I've been condidering the extended cab thing too. The best method would to be find a panel truck body and graft the rear of your cab to it.
I don't have the skill set, either. I'm not afraid of cutting and welding, it's the end result I'm worried about. I may end up with garbage for a cab.
Have you also considered adjusting the pedal throw?
It looks plausible to modify the brake and clutch pedal assembly so you don't have to lift your leg to your chin.
About moving the firewall, I did see a link awhile back where someon extended the cowell area for more leg room, I'll do some digging to see if I can find it.
Here's some eye candy in the meantime
https://48chevy.shutterfly.com/
