Keggle Welded Fitting Question

I am getting a keggle today with the top cut. I have the means to get a fitting welded into the base of it but have no idea what to get, and what size hole to have drilled, etc. What diameter and length should the pipe fitting be? Do I need the pipe to be threaded at both ends? Does the dip tube have to be fashioned onto the pipe before it is welded? Anything else to consider?

Sorry for my noobishness, I spent a long time searching for this information this morning and never really found any info surrounding this exact question.

Thanks!

the hole for a 1/2 coupler will be close to 1". I got mine from buyfittingsonline.com and they were great to work with. The step bit I got from harbor freight was worthless as many have found out. I ended up using a 1" hole saw and cleaning up the opening with a die grinder. Make sure you have your fittings on hand before you drill!! I have a set of digital calipers I measured the fittings prior to drilling, with chinese product you can never be too careful. Good luck!