I did a little more research. I'll cancel the sram cassette I ordered, or try to sell it or something (it was only $20, so no biggie). It seems like there should not be any spacing differences between the 7-speed cassette vs freewheel (shimano), so I should still be able to use the 8-speed ultegra bar end shifters and an XT derailleur. so... now my drive train is looking like this

phil wood 7 speed touring freewheel hub
7-speed cassette 13-34 or whatever I can find (should I bring an extra along on a long tour?)
8-speed ultegra bar end shifters
XT "shadow" rear derailleur
sugino xd cranks or something similar ~26-36-48 (opinions?)
compatible front derailleur (tiagra?) (XT won't accomodate 48t)
7/8 speed sram chain