318, made changes, no gain?

Riddle me this Batman...

I have a '65 Barracuda with a mild 318.
- 9.0:1 Flatheads
- Mopar Perf 484 cam
- 360 heads
- Thorley Headers
- 3.23 gears

I ran 14.30's-14.40's fairly consistent last year with this set-up. Changed gears to 3.91 and went from 22" to 26" tires for the 1st race this year, really no change. (Pulled a 14.09 but weather was awesome, adj. altitude was 578 ft.)

Changed out the tranny (was running the original in it yet) it's from a '68, still a 904 but with a taller 1st gear, Griner manual valve body (works awesome and sharp) and a 4000 stall converter. I ran anywhere from a 14.60 to a 14.90?!? (Adj. Alt was between 1800 & 2300 ft)

Question here is, is it just too much converter for the motor.
Is there a way to build this a little more consistent, I seem to get wild numbers out of it sometime. What should I be looking for that can throw a really slow number in there or why it will progressively fall off during the day.

What can I change here to get a nice mid-13, solid Sportman class car?

I'm not looking for crazy fast here, just the best, least expensive way of fixing the problems and putting me into the class I want to run.

Thanks!
KC