Have you checked the ignition Dwell . I assume the shop you took it to replaced the points and condenser and set the points but maybe not. i have some points in stock you are welcome to I converted all my 351's to HEI ignition.
Replace the points or checked them? You should check the dwell [or point gap] sometimes they will slip . Give me a call and I will walk you threw it. 678-227-8833
So i replaced the fuel filter and coil last week and this past weekend the boat performed well. The only hiccup, which i have no explanation of was that it died while going about 30 MPH. Wasn't running rough leading up to that or after, just died. Started it right back up and it ran fine the remaining 15 minute ride back to the dock. At this point we'll just have to see how it goes.
So I haven't had any other issues after running 2-3 more tanks of gas through the boat. I'm thinking a nasty fuel filter was restricting flow.