| A roof skin is relatively easy to replace...
Set the car up on jack stands in a level position and don't move it during the process.
First the sealer in the roof drip channel has to be removed to expose spot welds. The sealer can be scraped out with a screwdriver after warming the channel from underneath.
Spot welds around the entire perimeter have to be drilled out... a spot weld drill with adjustable depth setting is the easiest, but a spot weld cutter bit in a drill works fine, just slower.
Another option, if you're good with a grinder, is to grind thin at the spot welds & then pop them... you must have the finesse not to grind too deep however.
I like to do a "rough cut" around the top first... this way you have an easier to manage piece to pry up after the spot welds are broken loose.
Installing the replacement top is simply a matter of aligning and clamping it in place, then welding the spot welds back together.
You may want to consider bonding the replacement skin on with panel adhesive. If you do, you'll have to trim out the replacement panel without drilling the spot welds.
Doing it this way will save hours of welding & grinding and give a cleaner finished job.
I'd still suggest welding the roof seams solid however...
Also, take advantage of the top being accessable from both sides before installing... any dings or dents should be worked out at this time.