Don't be mislead by the 3/8" size...Its not translating to the opening....when it comes to pipe fittings, a 3/8" pipe thread has an outside diameter of just over 9/16".....if you got a 1/2" barb to a 3/8" male pipe thread fitting the pump wouldn't have any problems...it may choke a bit of the flow depending on your setup, but it should be ok.
With all due respect, that pretty much goes against everything your user guide says wrt providing a non-restricted path to the pump input. And who cares about the OD of a 3/8" pipe - it's the ID that counts, and a typical 3/8" brass npt pipe end has an ID of 1/4".
When the guide advise against having a 1/2" street elbow right at the pump inlet, it's hard to understand why you'd give your okey dokey to using a 3/8" pipe there...
Cheers!