Got myself a 19mm Axminster handy reamer for modifying my Sierra uprights to use with Maxi ball joint. What size hole do i need to ream out? Or how much of the ball joint thread should stick out of the bottom hole?

This job is probably best left until you have the lower ball joints to hand

You are more or less limited buy the length of the tapered section of the joint that you are going to use, if you push down on the rubber boot of the joint you'll be able to see where the taper ends.

Just ream a little out at a time untill you have enough thread protruding so as you can fit a flat washer, nyloc nut & still have a few threads showing once the nyloc has been tightened.

I have done several sets of these with no major dramas

A word of caution, it is not uncommon for different brands of joints to have differing lengths of the threadded section of the joint.