Wish I had a GN when I turned 16.... wait, no I don't. I'd have killed myself. I had a Plymouth Acclaim 91 w/ a 2.5L 4-cyl. Then that summer I bought my black SC 5-speed. Got enough tickets with it. Its only now about as quick as a stock GN, maybe a few tenths slower.
Anyhow I can't wait to hear you beating both these guys. Why is it V8 guys run there mouths so bad and can't believe you even want to race with a V6. I have schooled a few Z28's and many 5.0 and 4.6 GT stangs. They just talk **** and accuse me of either have a supercharged 5.0 (thats what most thing I have), or running nitrous. It pisses them off more when I pop the hood and let them count plug wires and look for a nitrous line. One guy made me pop the trunk and lift up the spare tire cover, he was convinced I had a sneaky pete system on the car. God forbid his 3.73 geared, headers, x-pipe, flowmasters, and mac intake equipped 5.0 notchback got beaten by my SC. I even let him drive my car and I drove his and he beat me in his car with my car. Then he finally came around and asked if I wanted to sell it.
Hope you got some mods, a 400 poncho with a cam, exhaust, gears, and some tires could run into the low 13's. You gotta remember, back in 67 they didn't have low compression EPA happy V8's either, they had some muscle. **** a 426 hemi was 425 hp, 440's were laying down 390 same as 03 Cobra's. I'd think his 400 is at least about 350-360 hp. I'd take a late 60's mopar over almost any other car from its time. But I don't think the poncho will be a pushover. The chevelle is heavy, so you can probably get him, but I'd be worried a bit about the T/A if you are stock. If you got high octane, exhaust, and slicks, you can up the boost and might win if you get the launch. If you are stock, I hate to say it, but you might get owned.