Should I butt-weld or flange the joint on the front fender patch? Got the OPG patch panel, and it looks pretty good. Getting ready to start, and looking for any good tips.

A fender path I would Butt weld... but if the fenders off the car where you have access to the back. an under lap p(patch sean under the fender and seam seal it after all your work is done ) can work out OK.
I still vote for the butt weld
trim the fit .then trim and fit...The you double check your fit again B-4 you flip on that welders switch :D

Lap welding a flange is alot easier, just harder to finish. The key questin is how good of a welder are you? Also look at the equipment you have, flux core butt welds are a challenge. With a small wire and good gas mix practice on small scraps. A good stitch welded butt joint will be a much easier fix.

I butt weld when ever possible, easier finishing with a finished repair that isn't detectable. Also use only as much of the patch panel as needed, most patch panels are kinda crude in shape and size so I usually cut out only what I need or make my own. If you do lap any joints make sure to treat the seam on the backside or it will collect moisture absorbing dirt/dust and will fail down the road.