Some positioner ideas:
Try to make it like the small table top units commercially available. it's good if the turntable can be horizontal and flip all the way down well past ninety degrees. On this turntable/faceplate you can mount your three or four jaw chuck, work, etc. You really need a hole down the center of the rotating shaft. You can use a full floating truck rear end hub. This centerline hole would be minimum 1/2 inch up to 2-3 inches in diameter. This allows for hoses, long work, etc. But be careful, no arcing through the bearings. The truck hub is super heavy duty but it's cheap. Try to use a DC/PM motor with a gear box. It's good to have maximum RPM of around 15, and low speed super super slow - .05 RPM. With all the torque you can possibly get. You need something to drive the DC motor, possibly on a separate pendant with an on/off switch, forward/reverse switch, two 10 turn potentiometers and a high/low switch to switch between the two potentiometers. This allows you to go fast to your next location and flip a switch to slow for welding. To ground your rotating shaft you need a machined round smooth surface to press against, either with a curved spring loaded piece of copper or a braided copper grounding strap with spring tension purchased from McMaster-Carr.
I wish you the best.