I had a 74 Dart Sport with a similar built 318 and it ran 13.90's... This was back in the early 90's and I loved that car... It was my Mustang 5.0 killer...
That's funny. When I had my 71 Swinger, my cousin Mike had an 86 Mustang LX Notch w/H.O. 5.0 and 5 spd. Also had 3.73 posi in it.
One day we went playing around and the cars were dead even. We stopped on the side of the road at one point and he said "Watch this."
Popped the hood on my car and he adjusted the timing on my car by ear by having me hold the rpm at 2000 and advancing it till it increased in rpm then decreased then set it just at the highest point in rpm before it would bog down again.
We then got back to it and every time my ragbag low compression 318 w/904 and 3.55 gear would pull away from that Stang, EVERY time! He had a big grin on his face and would just laugh, lol. He's a Mopar guy through and through and bought the Stang when it was close to new and used it as his daily driver/mpg getter, lol.