Grinding the underside of the fork crown

I very much care about matching the original geometry. Using a crown with a larger height gives the top tube a slight slope, very annoying to me.

how much extra length are we talking here? Because the toptube angle change from even a large fork length adjustment will be barely perceptible.

as a general rule, 20mm of fork length equates to around 1 degree of angle-change on the toptube.

I would argue that even a 1 degree angle is imperceptible by eye and this is using our extreme example of fork length change.

In this situation, realistically you'll be grinding out the fork to achieve somewhere in the region of 5mm fork-length change. This is a quarter of a degree of angle change on the toptube.

You should listen to your builder and trust them. Because right now you're focussing on something that is inconsequential and irrelevant