No top end power, need help
Unless you have one wild - *** cam, that thing should do WAY better than 13. I don't drive "carefully," and my 67 got 17 and some more with
(now 318)
360, 727, 3.89 Ford 9" rear, 28.+ tall tires, about 3300 at 70 mph
Little cam "about" like a 340 stick cam, Ed performer, Ed 625-650 carb
Letting off and "getting more power" might be too lean, too much carb.