cleaning up the slant six head

The Chebby 302's had 4.00" bores just like the Fords do. Both are 4" bore x 3" stroke. Cylinder volume may be close to the same as the 225, but the bore & stroke are not.

Any given port will have a max RPM that it will support for a particular cylinder volume. It simply chokes off the flow if you try to go above that RPM. You can increase the RPM by changing the pressure delta (turbo or supercharging), but that is about all that you can do without revising the port. A bigger valve on the same port is only going to buy you so much, and it could hurt flow by introducing turbulence where you don't want it. A high flowing port on a low speed engine will not work very well. I deliberately chose a small port head for a near future Chebby build because it is going into a 4WD. That engine doesn't want and can't use 300CFM ports.

There's a reason why modern cylinder head design employs supercomputers running Computational Fluid Dynamics programs (think 27th+ order Dif-EQ's), and those results are only as good as the assumptions made by both the program's writers and by the user. the rest of us have to cut and try. Even they eventually have to that.

I think the thing to do is to set a realistic max RPM, figure out where you need the Torque Peak and HP peak to be, and build the engine to come as close to that as you can. In the case of the Slant that upper RPM simply needs to be realistic in terms of it's cylinder head. Given that this is for a 4WD you probably aren't interested in turning 10,000 RPM anyway. So build to optimize max cylinder pressure from idle up to whatever your use dictates.