Anyhoo, here's some more food for thought. I turned the old block upside down with a head on it so you can see down the bore and more fully understand the issues with bigger valves and shrouding should you choose to go down this path as well. Below is a pic looking straight down the bore at what would be the combustion chamber. If you look at the outer edges of the valve you will see that they are almost right against the cylinder wall and are on the razors edge of the gasket. So, if you are going to go to with bigger valves, especially on the exhaust, consider how much room you have for radial clearance not just for the valve to piston reliefs, but also to the cylinder walls. As I understand it way back in the day chevrolet had this issue on 396's and maybe 454's. They ended up machining in a relief into the cylinder wall to help combat the issue. Now in this case you are looking at a 98mm standard bore, I have a 98.5 now and I'll have a 99mm bore on the next engine and I have to say that if you are going with bigger valves, a bigger bore would really help the issue. It almost doesn't make any sense to go through the trouble if the valves are getting choked up against the cylinder walls. The next issue is the diameter of the gasket, the stock gasket is not exactly perfect in that regard and I think that a cometic would be the way to go.

AV