Firing issue

I hope you adjusted the mechanical advance inside the distributor?

If you had 50* total on it without vacuum, then you were on your way to hurting the engine.

Having to mat the pedal when starting is also indicative of not enough initial timing.

Put a timing light on it at idle and see where it is set. Sounds like they did the total timing deal which may well be the correct total number. It should have 16-22 is my guess, but they backed out 12ish degrees. you need to get both ends of that under control.