I just did this on a '66. Some of the welds I was able to reach with a "tong style" spotwelder, but most I had to drill holes in the new inner fender and plug weld (aka "rosette" weld). The old panel is removed by drilling out the old spot welds and separating the panels with a thin chisel or in my case a very tough putty knife.
"When a woman is a really good driver, she is just about perfect."
- Raymond Chandler
'66 GT Fastback show car, nightmist blue, warmed up original 289 & T-5Z, 3.80 trac-loc, AC, PS, pony interior, Retrosound, rally pac
'66 Coupe driver, Bullitt green, 302HO w/ 351W heads, roller rockers, Holley 600, T-5Z, 3.55 trac-loc, collapsible column, tank armor, disk brakes, shoulder belts
'11 BMW 335i X-drive, 6 speed manual, all the bells and whistles