properly you need to lap the heads and block flat after having them decked. Seems like a lot of work given the stock gasket is known to work well. (Such as in Greg Dupree's 9.3 second car, IIRC)
Here a link to a guy on a supra forum discussion lapping and it sounds like he may be renting out the lapping block: La-Z-Link
Even if you had all the right equipment is may still be a matter of experience and individual skill to get good results. It seems to me if you used improper form lapping you could mess up the flatness.