mine is a 2 post twin hydraulic but similar principle was 3 phase. i left the original 3 phase motor but not wired up and connected a 3kw / 4hp single phase motor above geared down so i guess half the speed going up but same down as drops under gravity and lifts full weight no problem, so...
9 inch grinder with diamond blade preferably water cooled or at least a hosepipe to give some cooling . rider to that of Couse you need to check nothing load bearing above
if these are normal say 100x65 prestressed concrete lintels (they may not be ) ie normally used above an opening then they are layed correctly ie the width is 100 the height is 65 . paying slabs to equally distribute the load though and the base should be bigger
in my experience the thinner the harder especially car panels when your welding upside down on your back but welcome . there is a section on welding thin metal would post the link but cant find it right now
a multitool is very useful for floorboards at the edge and saw blades even of the same diameter can have different centre bores to best to check or some spacers to adapt
i used to work for gkn driveline and on some prop shafts and driveshafts the spline has to orientated to fit on think this is what above post is referring to
from the front plate the welder is 3 phase only which can be different voltages of 3 phase ie 220 or 380v its not been designed for single phase bit like a dual voltage 3 phase motor may say 220v but its 3 phase 220v if that makes sense