To limit the mechanical advance mechanism's travel, you weld the end of the slots that the advance arms contact when it advances. I found that chainsaw files are the perfect size to file and shape the slot with after welding. It's a bit of trial and error, but you must use a dial back timing light to see the effects your welding and filing have resulted in.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)