273 piston options
I was looking at valves online when it struck me that I have a set of half-ported 360 heads. Including valves. They're 1.88/1.6 in stead of 1.78/1.50 as stated above.
Turns out I got a set of brand spanking new exhaust valves when I bought the heads, I had completely forgotten about that.
There's quite a bit of difference (these are the stock 273 (920) heads, nice and blasted as of today).
With the 0.030" over cylinder bore I'll be getting, there's not much room, though. Pretty close to 0.050" on each side. Note that the chamber is much wider than the bore.