CASCO Thunderbird parts lists a correctly re machined ring gear for $75. Other Thunderbird vendors show them for about half that price, but I suspect they are part of a run made several years ago that, although useable, did not fit very well. As noted above by Rotorwrench you will need seals to reassemble the converter. If you choose to go this route be sure to get seals with square profile cross section as original. (some sell round ones, not good!) The ring gear will need to be pressed, sweated, onto the converter cover and then reinforced with spot welds as shown in enclosed picture.
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If all the teeth are still intact and the welds look like this..
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...and not like this..
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...then an alternative solution may be to use a replacement type high torque starter that engages the ring gear on the opposite side to the original type starter...
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I believe there is also a starter from an FE engine that engages on the still good side of the ring gear, though I do not know from what year or model it was fitted. Some research on a Crown Victoria or similar forum should yield a result to that end.
Good Luck. Very nice car.