Race car mystery-Help us solve PLEASE

I just fought a simular problem.
My Barracuda went from 6.40 @ 106 MPH to a 6.90 @ 99 MPH.
Thought it was the convertor, sent it in, tighten it up.
Same numbers. sent the convertor back, loosened it up.
Same numbers.

The problem was when the heads were ported, the porter had broken thru the casting on a intake runner at the push rod pinch. It had been brazed up & blended.
ran a leak down, worst cylinder was 2.5 percent.
removed the rockers & installed a plate to vacuum check the intake & heads.
I was suspecting a failed intake gasket.
I could blow into the hose that was hooked up & see it leaking out when I sprayed wd40 on it.

I have since put epoxy on it & going to run it this weekend to see if that fixed the problem.

That lost numbers seem like alot for valve springs.
Check the basics, leakdown to eleminate the short block.

I also had a friend with a 340 that no matter what you did, it would only run 11.20's every time.
I installed my carb (750 holley) same as his & changed the dist., same as his.
The car responded with a 11.56 pass.
These kind of problems will eat you up.