Then you have your answer, the motor will run perfectly with a cam with either firing order so there's not much else to be determined unless you start stripping parts off.

 

I missed the 1980 casting number, definitely not a 289, but could still be a 255. Casting numbers didn't change unless the block design changed. For instance, pre-1980 302's had an early 289 casting number (they used the same block), the block was redesigned in 1980 (different metal composition), and since 255's used the same block, it would use the same number. I have a 351w block in my garage, casting number is e4ae, meaning it was a 1984 redesign, but the date code indicates it was cast in 1988.

 

The 255 is not the same block. It has a 3.68" bore vs 4" of the 302/351w.

 

You are wrong everywhere here. The 289 and 302 used different blocks. A few 68 302's may have used the 289 casting, but after 68, all 302's used 302 castings. No 289 casting numbers were used on 302 castings. The 255 also used a different casting with a smaller bore than a 302 and thus would not have had 302 casting numbers. The 255 used the same stroke length as a 302, the difference in displacement was due to a smaller bore size. There were several 302 castings used in the 70's, not just a single casting.