So from a quick glance if you are correct on the GPH of your waterfall box it looks like you can easily change to a pondmaster 700gph pump.
This will use the same tubing / fittings you installed for the 350gph pump, anything larger pump wise and you will have to change out the tubing.
And at 3’ head height the 700gph pump will still provide 460gph.
this place has really good pricing on Pondmaster pumps:
http://www.petmountain.com/product/aquarium-water-pumps/11442-503230/pondmaster-pondmaster-magnetic-drive-utility-pump-for-ponds.html