Firing up a new engine combination is always nerve wracking, and trying to watch the trans fluid level while you are doing it makes things worse. I would suggest getting the trans filled properly before you actually fire the engine. The way to do that is to pull the plugs, put your 4-5 quarts of fluid in the trans and make sure there is oil in the engine, hook a battery charger up to the battery and spin the engine over for 20-30 seconds at a time. Make sure you can see the trans fluid level on the dipstick before you start. Once you start spinning the engine you should see engine oil pressure pretty quickly, especially if you have pre-oiled. Make sure you see it; you don't want to be spinning the engine over for an extended time without oil pressure. Crank for 20-30 seconds then stop for a minute, and repeat; this approach will keep you from overheating the starter. While you are doing this the front pump in the transmission will be filling the torque converter, valve body, and cooler lines so after you have done this 3-4 times check the transmission fluid level. It will have gone down, so refill and repeat. A C6 will take a lot of fluid, especially with a stock converter, so keep doing this until you've got around 8-9 quarts in the transmission. This is enough to prevent damage when you fire the engine for the first time. Then install your plugs and you are ready to fire. Once you are up and running for a couple minutes, start checking the trans fluid level and add more fluid as required.
Good luck on the startup - Jay
