All #48 heads had big valves.

All the open chamber high compression heads were from '68, '69, & '70. So, that limits your choices to heads from these 3 years. I have never seen any open chamber high compression small valve screw-in stud heads. But, that doesn't mean they don't exist.

If you look thru the head info at this first link, you'll see that it shows that some '69 428 engines came with both #16 & #46 heads, which had small valves & screw-in studs. I don't know if this info is correct or not. Online info differs, and we all know that not all online info is accurate.

http://ronspontiacpage.com/reference-pages/stockheadspecs.htm

http://www.bassettracing.cz28.com/custom3.html

If you can't find any heads with small valves & screw-in studs, I'd say the cheapest way to get some would be to find some good #15 #16, #17, #46, or #47 small valve cores, and have the screw-in stud conversion done. Only other choice I see is to use big valve heads & cylinder chamfers in the block.

One option might be to find some of these heads that have already been set up, with the screw-in studs. You might find some for a reasonable price, over on the PY forum.