Sluggish, hesitant acceleration ('76 Ford F-250 300 6 cyl.)

I've recently reinstalled a stock Carter 1 barrel carb on my '76 Ford F250, 300 c.I. 6 cylinder (w/auto trans). The engine runs very well with one exception. When climbing up to freeway speed the engine goes through a sluggish hesitation till about midrange through each gear. As the engine nears normal operating temp the sluggishness decreases some but still is there. As a test I accelerated more aggressively to freeway speed and that makes the hesitation much less but of course driving it that way full time is out of the question. Once cruzing at freeway speed it runs fine. The idle is fine and the electric choke works as it should. The plugs are new and the air filter is new. I checked the vacuum to the distributor and that checks out. Any suggestions will be greatly appreciated.