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I only ever had one set of headers - Sandersons, on a y-block, in a previous 59 I owned. I would think you’d need to have someone with the same motor/set-up as you, that has personal experience with a particular brand? Could be a long wait for someone to comment here, that seems not to be a real popular choice in a Slick. Good luck.
You might try asking in the engine forum. Small Block V8 (221, 260, 289, 5.0/302, 5.8/351W) - Ford Truck Enthusiasts Forums (ford-trucks.com)
The main problem with long tube headers for small blocks is that big front crossmember and the engine perches in these trucks. I had to build my own and have them coated when I put a 351C in my truck in order to keep everything inside the frame rails. Not to mention notching one of the crossmembers.
What headers will fit on a 1965 F100 with a 351 Windsor?
I just swapped my 240 for a 302 crate and had the same problem on my 65 f100. I ended up going with Sanderson FF2s. It was the safer option for the passenger side frame and they did fit. The one problem I had was I bought a crossmember back exhaust kit from Pypes. I had to take it to 2 different exhaust shops to find someone willing to make the intermediate pipes. It's extremely tight.
good luck