318 capabilities??
Hot Rod Magazine did an article years ago using a junkyard Duster with a 318 and kept spraying it until they blew it up. Like Hypermite said...all it was able to take was about 150, after that, it went boom.
I have a very similar set up to yours (71 Demon, 318, .030 over, 8 3/4 with 3.91 and a mini spool) and have been as fast as 8.32 in the 1/8th on motor, in really good air. Most of the conversion charts out put this right around the 12.95 to 13.05 range for the quarter. Hope this helps and good luck!!!