I am swapping a 351w into my 67 cougar. It came with a 289 and I am not sure what style after market header will fit. This is also a factory A/C car. I don’t know if that matters.
Headers that fit a 289 - 302 will work on a 351W in the 67 - 68 . I used headman when i did my old Cougar if you have PS you just need to get the adapter that goes to the frame to drop the PS slave cylinder. Shorty headers should work also. The engine bay is the basacilly the same from 67 to 70 so you have lots of choices since the 351 W came in 69 - 70 cars
Thank you
Hedeman offers a mid length header for the 351W that fits the '68. We used them on my son’s car with out having to relocate the PS ram.
This is the header drop bracket
https://www.ebay.com/itm/192668756772?ssPageName=STRK:MESELX:IT&_trksid=p3984.m1555.l2649
I’ve done this 351W into a '67 Cougar swap 3 times. I used Tri - Y headers from https://cobranda.com. Fit great.