3927186 heads
 

can the small valve 186 castings be converted to the large valve

Absolutely commonly done.

Don't forget to unshroud the intake valve area in the combustion chamber, otherwise just stay with the 1.94's as they will flow better without the cut.

Try to find pictures of the original Z28 LT1 heads and copy them. It's more than just putting in larger valves. The combustion chambers also need to be modified.

Will it run any better or worse with the small valve heads on a lt1 350

I doubt you will be able to feel any difference

If you are running exhaust manifolds and full exhaust, don't bother.