Really depends what he is doing. My mustang handles damn good with 15" rim/tires
If this is just a street cruiser keep it simple.
Make sure front end is in good shape. Convert to disc brake if that's what you want
1" bars
I like fresh leaf springs too. Xhd from espo will do
Good shocks.
Those few things will help big time.
Second stage could be.
Frame connectors
Front and rear sway bars.