1.5" is pretty big for 350
I used 1.375" OD on 378cc and that was mainly for 'looks' (it's 1/8" smaller on ID but inch and quarter is still pretty big for required flow)
As you know, I made a 'step' pipe, 1.375", 1.500", 1.625" and 1.750" after collector (all outside diameters)
Primaries are a bit long but it gives a real good spread of power and allows real good flow at high rpm (I would probably break it or me if it revved higher ;D )