I used a couple different brands of headers when I put a GT390 into my 59 Ranchero and later into a 2 door 62 Ford wagon.

On the Ranchero, the right side fit with no problems.
The left side required the two front primary tubes to be altered for clearance.
An easy job that requred cutting out a short straight area and substituting a piece of pipe with a couple curves on the front two cylinders.
The curved pipe built from a U-bend.

When I stuck the engine in the wagon, the right side fit with no problems and only one pipe on the left side needed to be altered.

Both sets of headers were for 390 into Mustang.

An old trick, get a set of headers for the smallest car that particular engine was in.
Many times, the headers will drop straight in with no alterations required.