I'll give you all of the factory part numbers for the "DODGE" letters and their retaining hardware that you need for the rear lower panel on your 1968 R/T and also an n.o.s. parts dealer in Canada that I've done business with several times over the years. Even if they do not have the parts you need in stock when you call them, they have many worldwide sources for n.o.s. parts, so they should be able to find them for you. I will say however, they may not be the cheapest source for n.o.s. parts that you might find, but they are very good folks to deal with and as mentioned, will either have your needed parts in stock, or will find them for you and how much is your time spent looking for parts worth?

Please make sure to have the part numbers I gave you below at your disposal when you call them, as they will appreciate that. Saves time for all involved.
Part numbers:
Part number: 2840 573
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: letter "D", name letter, deck opening, lower panel
Number used per vehicle" 1
Part number: 2840 574
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: letter "O", name letter, deck opening, lower panel
Number used per vehicle" 1
Part number: 2840 575
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: letter "D", name letter, deck opening, lower panel
Number used per vehicle" 1
Part number: 2840 576
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: letter "G", name letter, deck opening, lower panel
Number used per vehicle" 1
Part number: 2840 577
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: letter "E", name letter, deck opening, lower panel
Number used per vehicle" 1
Part number: 6027 352
1968 Factory Parts Book section number: Illustration page 23-248 and Illustration page 23-249 (exterior view - Coronet R/T (W-S) 2 door hardtop)
Part description: nut and seal
Number used per vehicle: 10 (2 per each letter)
Contact information: Collectors Auto Supply Inc. (located in Canada)
Collectors Auto Supply Inc.
6727 Thorne Road
Peachland, B.C.
Canada
V0H 1X9
Hours: 7 am to 4 pm Mon-Friday and Saturdays by chance. (west coast time)
Toll free phone number: 1-800-414-4462
Web site home page link:
http://www.collectorsautosupply.com/
I hope this helps.
Richard