This kind of question gets asked here all the time. To give you the best answer we'd need a lot more info than simply what brand of manifold, carb, etc. that you have now. There are many things that make a car run right - meaning the correct combination makes a world of difference in performance. If your intended use is going to be 90% street and just want a little more then you'll need to make the power down in the lower to mid RPM range. If you want a real performance car out of it you'd probably want it higher in the mid to upper RPM range.
Everything from the carb size & type (600, 650, 750 CFM, etc. and is it vacuum or mechanical secondaries), intake type (Performer, Performer RPM, Victor Jr., etc.) need to be knowm. Heads can make a big difference in performance if they are the right ones for what you need. If anything in the combination is not right the performance suffers. Stock ones usually aren't worth much for performance even if they are "fresh". You need a head that flows in the RPM range you intend to run the motor in. Too small or too large can hurt power if the engine isn't set up for it.
Get as much info as you can and the answers will flow. Even part numbers if you can find them will be helpful in the long run. You say the heads are fresh but we have no idea what heads they are so it could be as simple as replacing them with something better or the carb might be too small or too large. If the current compression ratio isn't right for the cam, heads, intake and so on the entire combo is off.
Stock gearing can be a number of different ratios as well so that needs to be known. Sometimes the rearend has the tag that has that info but if not just jack up the rearend and count the driveshaft turns to make one revolution of the tires. You'll need help on that one probably. That will get you close enough though.
Everything you have may be just right but if you're running say a 2.80 or 3.00 gear like many Ford 8-inch rears had, your performance will suffer. Let the info sharing begin and you'll get better help.
With all that said I think I'm taking the Falcon out for a short cruise!
John
