Hey Atec, replied to your PM before but see you are in good hands here. What scythe mentioned is what I did. Scribe, cut and grind. With a few additional steps.
Bear in mind I had one end of my horizontal keg cut off for access to the internals.
scythe wrote:I think cut and grind will be the way to go.
Trim the vertical keg where you want it and tack it to the horizontal keg.
Make sure it's centered, square and plumb as you wish.
Then just transfer the diameter down onto the horizontal keg.
Cut and grind your hole to suit, slide vertical keg into hole fully (check squareness again) and scribe profile.
Cut and grind vertical keg to profile.
When fitting the two kegs back together, noticed that cutting the profile into the vertical keg took it out of roundness, was a pain trying to get the joins lined up evenly and get it square and plumb at same time. Ended up sliding vertical keg back into horizontal keg with 10-15mm sticking proud to the internal. This sorted out roundness and allowed me to get vertical keg plumb and square easily
Weld.
Then using flap wheel grind away the 10-15mm sticking proud back to the horizontal internal surface.
Hope that helps.