In practice, the 3/8" pickup has worked fine on my B based 496 run on road track up to 6,500 RPM and street. I do not think the bigger pickup has practical benefit in a relatively mild engine... though I agree less restriction in theory is better. The bigger pickup has drawbacks also like potential for block cracks and stroker clearance.

From your starting oint, I agree with Cab and others... you are already tapped for 1/2" so get a hemi re-pop 1/2" pickup and call it good. A bushing is just another potential failure point and I would avoid that.