Not sure where you get the 32mm figure from. The yoke bores are designed to take bronze cylindrical leadscrew nuts, as opposed to being directly threaded. On my Series 1 BP clone, the original X&Y leadscrews were 1-1/4" ie ~32mm OD. The actual leadscrew nuts themselves were pretty much 39mm OD and the DFU2505 ballnuts are 40mm OD, so I only had to bore the yoke bores out about 1mm on their diameters.

As for fixing, there's really no need to use all 6 holes. I only fitted two M6 screws and there was enough meat for me to drill and tap the holes. If you look at the tensile load that would cause an M6 bolt to fail, you'll see that this wouldn't be the first link in the chain to break.

Here's a bit about the actual boring: https://mightyshiz.blogspot.com/2018...or-larger.html

And fitting the thing back on the machine. Note that the Y axis ballnut may foul something (I forget - possibly the knee lift?), which required some angle grinder action on mine.
https://mightyshiz.blogspot.com/2018...allscrews.html

Murray