Apart from that their AT options are very lackluster sans E8. They really need some sort of meatshield tank, or at least for bazookas not to suck, especially considering they cost fuel to get. Or maybe a heavy AT gun, did they US deploy some of those? A stolen Flak 88 ?
A meatshield tank for the USF would certainly make things easier. In the absence of those, it seems you have to make do with M23 Smoke Grenades and the Stuart's abilities to hold enemy tanks in place and prevent them from returning fire.
As for heavier AT guns: the US employed the 90mm M2 dual-purpose gun. It was quite similar to the famed German 88m Flak 36 gun. I could see it as a doctrinal emplacement in the future.
There was also the M5 Anti-Tank gun, which was based on a 3" anti-aircraft gun and started seeing deployment in 1943. It's basically the same gun that was used on the M10, however.
They didn't use anything heavier than that, so... maybe a reverse-lend leased 17pdr gun? Again, would be conceivable as a doctrinal emplacement, like the Brits had in vCoH or the Pak43 that's in the game now.
Edit: There was also the experimental 105mm T8 gun, which could penetrate 210mm of armour at 1000 yards, but weighed 8 tons (definitely not movable by the crew) and came too late to see action in WW2.