New main HVAC plenum duct fitment problem

[TA301]

5 hours spent on trying to fit the plenum duct [mounts right under the glove box] It just does not fit. The mounting pad where the two holes are for installing it on the dash brace, are just too thick. Have any of you installed this new reproduction duct? I tried my original duct and fit in no time.
Problem is the holes are, as usual, cracked. This is preventing me from completing the dash end of the restoration.

https://www.firebirdcentral.com/1970_1981_Duct_Housing_Vent_Under_Dash_RH_AC_p/duc-2015.htm

[5th T/A]

I don’t have any experience with the reproduction part. Mine was cracked where the two bolts connect to the bottom of the dash, right below the glove box. I used a fiberglass repair kit available at most auto parts stores. The repaired area came out ok but the finish is semi transparent so I sprayed that area with a little semigloss black paint. It’s a cheap fix.

[TA301]

I "repaired" my original with JB Weld black plastic specific adhesive. It does not work. I did manage to buy a mint GM original from ebay and will install it. It seems nobody here has ever installed that reproduction part, except me. Surprising considering vast majority of the originals are in disrepair.