What's an 739 ? Shifters or mech ?
Sorry these shimano numbers are all new to me
I have the xt 8 speed mech , no shifters yet so I may sell this cassette off cheap as its missing one ring, great condition
m739 is a shimano xt series number,
have a look her it will make more sensehttp://mombat.org/Shimano.htm
my point was a supposedly designated 8 speed rear mech such as the xt m739, will work quite happily with a 7,8,9 or indeed 10 speed cassette.
its is the shifters and the cassette which decide how many speed bike it is not the mech, so your shifters and cassette must be the same speed.
seeing as you have an 8,9,10 speed rear hub( all the same) you can either go 8,9, or even ten speed. seeing as it is retrobike and you already have the hub,8 speed might be your best bet