4.25" may be possible in a 302, but 4.125" is the largest I've heard of, and even this may compromise wall rigidity.

Here's the "12 Port News" section on the Howard head:
http://www.inliners.org/12_Port_Story/pt_six.html

The OEM rocker shafts are .791" like the 235/261, but longer.
Crower makes roller tappets (66259-12).

270 and smaller GMC motors have the crankshaft centered under the cylinder bores and the pin offset in the piston (as is normally done). The 302 crank is offset and the pins are centered.

A cam vendor gives the rocker ratio as 1.65:1, but since the G.M.C. accepts 235 rocker (1.477:1) this may be a typo.