Torque converter issue?
727 or 904 Torque Flight? Either the converters (Stock) are in the $300 range.
You may find a local tranny shop,or even Jegs or Summit if you can't find local.
At one time Jegs had 25-2800 stall speed 727 converters (Jegs brand)for $300-$400.
Haven't looked for awhile.