It looks like $450 isn't too far out of line. Ebay has a 71 automatic core (4973S) for sale for $299, and a 71 manual core (4972S) sold last week for $271 (
https://www.ebay.com/itm/Mopar-1971-...sAAOSwkelb~0Pz).
I'd be real interested in the HP delta you find between the 71 model and the later TQ's!