What will my 68 Dart 408 run in 1/4 mile?

Or you are a streeter, you can just do what I do; namely ignore the ET and be happy with big-speed. If you spend enough money you can eventually whittle your ET down.

Assuming 3450 race weight and 485hp, your W/P is 7.11 and the Speed that goes with that is about 122 mph.
Now an unlimited SS chassis might turn that into a low 11/high 10.......
but a streeter might leave .5 second or more in the 60ft, and the 3.55 might lose another .4 to the traps, so your probably looking at a very low 12. Then factor in a street tune, and maybe lo to mid 12's could be in the cards for you. If you short-shift, or you over-rev, or you just can't shift that beast worth xxxx, well then there's that.....
But if you can make it hook, well then, 11s are back on the table.
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But I'll tell ya, if I saw something close to 120mph on my slip, I'd call R done, and go get me some track-food, cuz I ain't spending all summer whittling the ET down. I'm quite content to know that my chassis sucks,lol. Heck I was happy with a 93 Eighth, and a 2.2plus 60ft..............