I can confirm for a fact that it will fit 1973 full size models including 1973 New Yorkers. Repaired @73 New Yorker heater box about a year ago. The seals were a perfect fit.
Removal of the blower housing under the hood will help you in the reinstall of your heater box later. If you haven't already order a set of foam gaskets from DMT, ( @detroitmucletechnologoies). They don't cost that much and will greatly improve Air direction flow and performance.
To aid in the removal of the heater hoses . remove the right inner fender. It will also give better access to the heater box bolts and screws. Following the info in @ayilar thread will help you.
Quarter Window seals are available from Detroit Muscle Technologies, they should have a booth at the Chrysler Performance Show Saturday. That would be an excellent opportunity to confirm availability and take them a sample for comparison.
Welcome from Indy. Please post a photo of your C Body. If you are not aware the 28th Annual Chrysler Proformance and Trade Show is coming up March 3rd & 4th. Lots of swap meet vendors there.
Welcome from a fellow owner. Come on out to shows left this year.To find where the local car shows are log on the internet sight oldcarsonly.com. The search bar on the left will take you to the current month for car shows. There are cruise-in notices also. Hope to see you there. To confirm...