If you didn't prefit everything you may run into some surprises, but let's hope not. Over shimming of the body (or sagging running board braces) can make it tough to pinch the welting at the front half of the fender.
Keep in mind that originally Ford mounted the fender to the body before it was dropped onto the chassis which already had the the front fenders, running boards, and aprons in place. They simply pressed the fender up tight pinching the welt in place and leaving the front end of the welting to be tucked against the apron when it came down.
In our case we're typically adding the rear fenders last. Get it into position loosely with the nuts 1-2 turns from snug and fit the welting. To avoid any scratches you need to install the fender to running board bolts similarly, just tight enough that the bolts don't tilt.
I typically have the car on the ground so I use my upper leg just above the knee to force the rear tail of the fender upward and tighten the lower rear two bolts (from memory). Then with the front end of the welting formed into position do similarly to the front. However there will be more opposing tension there due to the running board. You may choose to use a little help with a padded block and a bottle jack. Do not over do the jack! Tighten everything and release any temporary supports. If all is well the welting will remain snugged into place.
