Small Block & Big Block Chevy Specific - donor motor for vortec heads

I am looking to swap my stock cast iron 76cc heads for a set of 64cc vortec heads and was wonder what cars/trucks should I look out for in the junkyards and the casting #'s

I also heard there was a certain year or casting vortec head I should stay away from but can not find the link now

current setup: 91 s10, chevy 350(8.5/1 CR) with stock heads and cam, edelbrock intake, 650 holley, headman headers, full msd with a 100shot of juice

Vortecs started in production in '96 truck 350's. Casting numbers to look for are 10239906 and 12558062. Some say the 062's are more desirable than the 906's, but others say there's no difference. I've got 906's on my 10.4:1 383 stroker and have zero issues.

I think vortecs are Vortecs, dont just look at trucks, the vans also had them, full size vans that is.