Early 351W heads can be ported to work fairly well - not as good as the best aftermarket heads - but pretty well. These folks do CNC porting on heads and have their flow numbers posted on-line:
Small Block Ford 289 - 302 - 351W "Early"

The biggest restriction is on the exhaust side where there's a ruddy great thermactor hump in the middle of the port. I'm in the midst of porting a set now. A few hours of grinder work makes a huge difference.