[kitcarbp]

Dirk, If you are overheatig when above 35 mph its either a water flow issue (not enough) or as FWB pointed out there is not enough air going through the core, (its going around it) shroud in front of rad to improve.
Do not put a restrictor in place of the thermostat, you want to maximize water flow not slow it down, this will make it worse. Putting the larger elec. fan in will only solve problems of low speed overheating when you dont have enough air velocity over fins and need to pull the air through core using high cfm fan . There is still something mech. not correct. (you said WATER FLOW IS ok). I dont think you are cavitating pump at highway speeds (2-3K rpm), this would happen at higher engine/pump rpms.

Initial timing checked with a timing light is confirmed to be what ? (between 10-16 BTDC is the range but depends on your combo and octane)

Engine builder should have shared exactly what INT timing should be on pump gas not race gas. I assume he set up engine timing for intended pump gas (92/93 octane yes ? Retarded ign timing will add heat to combustion. Too lean of a A/F ratio at steady state cruise is another root cause of over heating at speed.

Engine builder should confirm your carb was set up and A/F ratio checked while on dyno at steady state. Based on what you have now described, the 3,000 cfm fan will not solve this issue of overheating while cruising.