You have got a problem unless you cut the plate underwater or in an isolation chamber. Either way you will have a lead disposal problem with contaminated water or filter dust.
There are equipment that can remove the lead paint on the cut line and can be rented. It depends on your location whether you will be able to used the equipment or a licensed contractor will have to do the work.
If you can I would get a legitimate lead abatement contractor in for a consultation and see what they have to say. You may have to remove all the lead depending on the future exposure.
There is another possibility and that is to use a water jet cutter if the authorities will allow it.
If you can come back with your location that will help a lot.
Try using a paint stripping compound to remove the lead paint from the steel to cut. Any high heat source will vaporize the lead. If you have a large amount try an abaitment company. Low heat methods will still generate the lead waste that must be dealt with.
One of the most common approaches is to use a vacuum blasting set up and remove the paint along the cut lines. A touch / gas axe (or any other device) may then be used. If the structure is to be dismantled, the cleanest approach is to then send it to a steel re-cycle facility. This eliminates the paint disposal problem.
