Originally Posted by Plaintoast
lots of trucks came with casting 991 heads which was a 1.72 intake valve. there are other castings that did too, but thats a very common one.
Indeed there were- like to have fell out when I pulled the 350 SBC from a ca. '77 Blazer and found 1.72" intakes and 3/8" stem
1.5" exhaust valves- what you might expect to see on a 20-30 Series truck. I don't recall the c/n, darn it!
This engine seemed
to be the original to the Blazer- had never-removed exhaust manifold bolt locking tabs, the OEM thread lock still on the bolts that hadn't been removed before, all original ground straps everywhere, etc.
Found this HERE
1971-77 heads w/casting numbers that end in 882, 993, 487, 336 can have the 3/8" exhaust stems. 20-30 Series truck use.
Both small 1.72" intake and
3/8" stem exhaust valve on c/n's 261, 624 (likely one of these was on the Blazer I worked on).
Also 1987-'95 center bolt v-cover-type head c/n 14102191 can have the 3/8" exhaust valve.
EDIT- This could be a good engine to use the Vortec head on- it does require a different intake, but since you're already using a poor intake (IMO) an intake swap would help in any case.
There are $140 (new) intakes available for the Vortec heads from Professional Products, heads cost about $150 used (give or take), require some inexpensive mods to use w/lifts over .450" or so.
You can do a search on this forum for more info on the Vortec swap.