I don’t have an answer to your specific question, but Don Gamble did a rewrite of the guffy post processor. You can find a link to it in this post.
When you say “runs at 51mm,” have you taken a look at the g-code to see what size it thinks it is cutting? I’ve not used Fusion 360 for CNC work in a few weeks, so if this is a recent change, I’ve not seen it. Historically the big change to the hobby version of Fusion 360 was the removal of rapids. The result was the Z axis being driven at too high of a feedrate and therefore losing steps.