'57 3200 w/ solid axle. Tires or upgrades first?

[yossarian19]

The truck handles better than I would expect but it is still a little loose on the freeway, leans a little in the turns. I've got fresh(ish) shocks, wheel bearings, disc brakes, steering linkage & fixed-end spring bushings into the front end. Shackles are still shot to hell and no sway bars.
My dad is swearing up and down that the first thing I do for handling should be 17x9 wheels & tires, that the rest will seem unimportant by comparison.
I'm thinking that the chassis is the weak link, performing well under what even the 235/75/15's will do on a tighter front end.
So - who's right?
If I was going to spend $$ on making the truck a little more highway friendly, are tires or bushings & sway bars going to make the bigger bang for the buck difference?