GHS Precision flats seem to work fine thru body. They came stock on 70s Stingrays which were thru body, and I have some on a thru body short scale with no issues. Fender flats would be okay too, as they ship them on the JMJ mustang. They may not be long enough though on a 34 inch thru body (would have to double check their winding length).
Chromes are probably fine to use as well.
What sound do you want? Every brand of flat sounds wildly different to my ears.
Chromes for example are super bright (almost like rounds) when fresh, take ages to die down into thump territory and are very stiff compared to most rounds.
GHS Precisions sound very old school (probably the thumpiest flats around) in comparison with similar tension feel to same gauge rounds.
Labella are the "default" flat tone imo, as almost every famous flat player used these on classic recordings, if you wanna sound like Jamerson or Chuck Rainey, get Labella. Quite flexible, feel pretty close to equivalent sized rounds.
Dunlop are nice, they have some top end but some decent thump too, pretty low tension (less than GHS/Labella).
What are your stock strings? My Yamaha 734 came with 100-45 D'addario so I'll assume you want similar tension/stiffness? Chromes and Fenders in the same gauge are far more stiffer feeling. Maybe light gauge Dunlops or GHS would feel similar.