Funny, This is just about what I had in mind to do up mine. I am not positive, but I believe you can get away with not going up the entire side of the fender. I believe the area where the lower filler panel fits does indeed to be spliced in, but from what I can tell, the upper outside corner of the fender just needs to be massaged to fit the grill.

And again, I haven't test fitted my core support in the truck yet, But I do not beleive there was any huge differences in the mounting flanges to the fenders when I laid an '89 core support on top of a '78 core support. I think at most it was maybe some trimming and relocation of fender bolt holes either in the fender or core support.

I should really write this stuff down when I do things like this as I am much more forgetful as time marches on, LOL