I did my 68 front about a week ago.
Yeah kinda a pain - I couldn't get my fender upper front triangle parts off (the parts of fender that mount to bumper support).
Those crazy large phillips screws would not come out and I didnt feel like cutting them off ect.

Idealy I think you take those fender attachments off. With those off I 'think' you can put the whole bumper assy with lights in and screw the like 3-5 bolts from the main frame bumper bracket to the bumper assy on each side - then put upper fender brackets back in. **Note one large round caped bolt has to be fed into the frame bracket on each side and that bolt is put in prior to the lights being attached in the bumper assy as the lights cover that area of the bumper support. This capped bolt was held so its not flopping around by a locking ring/washer when I took mine off.

Since I couldnt get my fender brackets off I put the whole front bumper assy together "with the frame brackets on sticking out" and upper support spacers installed - and fished those brackets in and bolted up 2 large bolts from brackets on each side to the frame. I had to kinda bend those upper fender brackets up a bit when I fit in the bushing spacers that are mounted on top of the bumper support.

Anyway thats what I did and how I think it should be done (not quite how I did it) - man I hope I dont have to do that again.