One last go with 51

[sweet na]

Wait for a cool day or dense air and give it a baseline run. Then go up 2 jet sizes in the secondaries and if the mph improves, go up 2 sizes in the primaries and so on until it doesn't improve anymore. If the mph drops off with the first jet change, drop 2 jet sizes and see if there's an improvement. If there's little or no change, then your jetting was pretty right. Also make sure you have a large, low restriction air filter.
If the car's having trouble hooking up, try dropping the tyre pressures a couple of pounds until you get an improvement, assuming you started with over 20 psi. Depending on the tune of the motor, you could also try advancing the timing 1 degree at a time and see what happens, especially if you put some 98 or 100 octane fuel in it.
Probably way too many things to try at one meeting but have fun trying.