I believe those would be small ports on the 248. Check out Patrick's Antique Autos website, his on line catalog has lots of info and photos of GMC heads.
http://www.patricksantiquecars.com/
Plus I'm sure others will be along to chime in.

Last edited by JiMerit Boltr#43; 06/06/2014 3:39 AM.

It's easier to get forgiveness than permission!
1946 1/2-Ton Chevy
1953 Chevy 3/4-ton Factory Stakebed