I hadn't thought about special valves, I had assumed and intended that to turn down the heads of two of the valves was the strategy, until I came up against the query about fitting a "chunky" valve insert.
I have been a huge fan of supercharging, since I read as a child about how effective a blower is at macerating the fuel-air mixture. Even as little as 4psi transformed the torque of a humble Morris Minor, so when I read something in Autosport or Motor about "Twirl", I thought it needed looking into, but it doesn't seem to have caught on in much of the tuning world.
To cut the valve pockets in the pistons, I knocked up a Fly-cutter, inserted it up the valve guide and cut the inlets with the piston locked at TDC and Henry sucking the swarf out of another port. I then cut it down a bit and did the exhausts. Bit of a bind taking the head off each time, but I only held it in place with 2 studs.