I installed Flowtech 32500 Long Tube Headers on my 79 f100 302 c6 ,,which our frames are about the same but I know there some differences but here's what I had to do. ,
I had body off the frame , I had engine sitting in place with tranny and I had to notch that brace crossmember in your video as the drive train on our trucks sit farther to passenger side as its designed that way , may be because of steering box on drivers side but unsure.. You can measure off rear of tail shaft to frame side on passenger side an it is close to 11 1/2" while drivers side measures 14 1/2" from frame to rear of tail shaft , , its easy to see with body off frame ,,This makes it easy for drivers side but passenger is a pita , I also had to notch in two other areas as pictures show .
, I had it installed before that notching but it was causing tranny to push over so hard that you couldn't line up bolt in the tranny mount on crossmember , I could force it over with a jack off the frame but decided to notch instead an was glad that I did as it allowed me also to use my tranny lines with out cutting just some tweaking , you can see in pictures were I also cut small notch on top of fram down side an also notched the passenger engine perch , this all helped drop header in easier as well an gave better access to starter bolts , I also used Hedman 18804 Exhaust Pipe Extension 2.5" Tube 3" Flanges to get exhaust pipes back were they needed to be

 

Awesome thanks bro!
im gonna remove the engine and rebuild it and when i do ill notch it as you said.
Did you use a angle grinder or cutting torch?
I take it the headers cant be bent a little bit?

 

I used a cutting wheel on grinder an a cut off tool pneumatic , I plan on removing that cross member again and cleaning up the cuts a smooth out as my body is still off as I am getting it ready for painting , I did notice out cross members are different but same principle applies,, as far as bending header pipe not likely , you might make a dent in pipe for areas to clear but notching works good

 

Try loosening the motor mounts. Pick up the engine an inch or two. Drop it back down. Maybe it will settle in a different spot. But these engines were set to the right a tad at the factory.

 

I had the engine and trans raised off the mounts just to clear the starter and theres no side to side adjustment apparently. If the company wouldve extended the headers by 2in it would really help
as it is i can tell which cylinders are bad cause the paint on the headers has only melted on certain cylinders lol 2,4,6, and 8 are all bad apparently

 

May be bronco headers would have been easier fight.

My old long tube headers back in the 70 was that the engine had to be raised up 4''-6" to get them into place then lower the engine into the mounts.
The best part was they did not trap the oem starter an the way the rear pipe was position it never over heated the starter.

But it sounds like those headers were for maybe a 73 or later that had a wider frames.
Or they got mixed up as for what yrs or model they would fit since you had to butcher the frame some ..

Orich