Usally gm makes the heads and blocks are made by many manufactures. Brodix and dart can also get you heads.
They are LS based now will straight port design. You cna buy the heads but they are for 4.1 ls short stroke bores not normal 350 style engines.
If you want a nascar or nascar style engine you can get pontiac performance heads from gm performance or you can go 18 degree heads that are also ery good. 18 degree heads and intake will build one hell of a gen one engine. 800 hp is normal number for 18 degree heads.
Sb2 is also easy to get for a late block but will cost a pile of cash over the gen one upgrades. But with 12 degree valves they will make even more power with less rpm and better flow.
Here is a link to gm sight for race heads.