Does anyone know how much can be taken off? I have a set of heads that have a bad surface. Thanks in advance!

 

I've had a set of c heads milled until they had a flat spot on the center divider.

 

I have had them cut 0.190" to bring the center divider up flush.

 

I could be wrong but I do not believe the stock exhaust manifolds will fit if you mill it to the center divider? I thought I remember the manifold hitting the block.

 

I know for a fact that certain headers and maybe even some manifolds will hit the block with that much surfacing of the exhaust side. That will depend on the design.
All I'm saying to the OP is that there is no hard and fast rule. You do it, you test fit and then you might have to modify another component to adjust. That's the fun of car crafting.
If you have the heads off you could also use silicon bronze mig wire to fill in the gap and then surface it flush. Silicon bronze mig wire is a great filler and acts more like brazing than a weld.
Many ways to skin a cat.

 

I might have a set of usable heads. Only paid $80 for them. They are GA and it turns out they have pressed in hardened exhaust seats and big valves. The machinist said I may need new valves since someone chopped the tops of the valves flush with the rotators and the rocker tips are practically sitting on top of the rotators. I am going to try some retainers since they should sit lower than the valve stems compared to the rotators to avoid new valves. The guides are good enough.


I do have a mig welder but never tried welding cast iron, always heard horror stories about it