Mods question

Just the opposite. Stretching helps to keep the front end down because of where the engine is in the frame. If the engine were further back then yes it would change cog some, but not enough to really do anything. If your wanting to start modding your bike for straight track the first things that come to mind is lowering the bike, especially the front and swapping sprokets. Then you just work from there. When I launch my front wheel really doesn't touch the ground until about half way through second gear but its not more than about an inch or 3 off the ground either.

Practice Practice Practice. Its the only way to get your times down. The place you need to work the most is your 60' times. Everything else comes from there.

As for what sprokets I suggest depends on bike and what set up you have. If it is purely a drag bike then you need to take hp/tq into account on the counter sproket. Sometimes you actually have to go up and then just go up more on the rear if you have too much power. When my bike was set up purely for racing I was at 16/43 with stock being 17/40. Would of kept the set up I had but need the bike for more than just racing so went back to a closer to stock set up overall to keep it more streetable.