Did you check Hooker? They used to make a very nice pair of square port Mustang headers, not sure what all else...Used to use them on my Maverick small blocks...... Most everything else I build is some sort of weird combination and I just start with a pair of flanges and go from there......

Hmmmm...Aren't the regular factory 5.0 headers off the GT Mustangs a square port Bobby???? See a lot of them on eBay....