I was actually assiming I would get the heads that were made for the 4.030 block and work them to use with my 4.125 block. But how does it work the other way, if at all?

Can I use a bigger bore head on a smaller bore block?

 

Don't see a problem with it as long as you check the cylinder wall clearance to see how close it is. On older BBC motors they actually notched the cylinder wall to help open up the valve area from the factory.

 

I don't know if this matters, but I am using a 400 SBC block, not BBC.

 

I know, I'm just saying at one time it wasn't uncommon for the factory to do it on production motors so I don't think there will be a problem as long as your clearances are ok.

 

How big are the valves? You're only talking about .03" difference in bore size, that's like 8 sheets of paper.

 

2.08/1.60"

 

If you're saying the chambers were opened up to the 4.155 size, like he said, about 8 sheets of paper. I wouldn't worry at all about shrouding. Now, I was gonna put Sportsman II's for a 4" and bigger bore on a 307 with a bore of 3.875". That's about a .065" (on each side) shelf the airflow will hit. So in that case it would be ideal to notch the bore.
You're difference is way to little to care about unless you're getting really nitty-gritty.