Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
Here's something interesting, an ebay listing for a new CB250 head K0-K4 but notice in the pics that the chamfer is present, unlike AshimotoK0's head - https://www.ebay.co.uk/p/Cylinder-Head-Honda-Cb250-K0-k4-B4-Cl250k-Part-NOS/605341165I make the diameter on mine (excluding chamfer) about the same as AshimotoK0's 250 head. Have you measured the depth? Statfold engineering have said that they would have a go at making good my cylinder head but I might look out for a good condition 250 head first! It seems a lot easier to just machine a chamfer than make good on a chamber with a resemblance to the lunar surface!
It looks to me that you'd need about 3.9cc larger volume in the 350 combustion chamber to maintain compression, at a rough calculation. If you spread that between piston crown and chamfer volume then it's not that much material.