quarterbore: you can't turn a pre-ban rifle receiver into a pistol - only into a short-barreled rifle. By definition, a pre-ban receiver has been assembled into something, and unless it was assembled into a pre-ban pistol (they're quite rare), it can't ever become a pistol. The $200 tax stamp and Form 1 is to make it into a short-barreled rifle, since a pistol is not an NFA item, by itself.
If you were to put 'pistol' on the form 1, I think ATF would either reject it out of hand, or contact you to ask for a clarification of what you were trying to do. If you build a pistol configuration from a receiver originally assembled as a rifle, be it pre-ban or post-ban, you're making an SBR, not a pistol.
I see one possible gray area to exploit: The ATF guidelines say the receiver must have never been ASSEMBLED as a rifle. However, a receiver is pre-ban if it was set aside, stripped, with all parts necessary to make a rifle, before the date of the ban.
While this is clearly a pre-ban rifle in the eyes of the ATF, there might be a loophole in the language for a rifle kit determined to be pre-ban, but never actually ASSEMBLED. Try that one out on the ATF for size.