The hardware and hardware finish is a mix of what was mostly available on the assembly line. As the plates and associated hardware was reused (or supposed to be), the finish was in poor shape. What I have found on 65 thru 69 Mustangs is as follows:
Bolts - the bolt used to attach the bell housing to the engine on SB V8 engines was used from 65 thru 68, p/n 20546-S2. For 1969 thru 73 I found the bolt to be only used on the plate, no other application on Mustangs. The only documentation I've found is on the 71-73 Body Assembly manuals, AM0041 page 61 and AM0046 page 63. The short bolt is 20430-S2 which was used to attach the bell housing to the transmission.
Washers - the washers are also unique to the plates, 44730-S2 and some other off the wall type that the closest AMK item is a GM bumper washer.
Nuts - a real mix - 64-66 were the front bumper nuts, 374358-S8 (when you remove a rusty nut and the "protected" underside is clear zinc, the whole nut is clear zinc): 67-68 used many; 34446-S8, I6 trans mount; 55695-S2, rear bumper; 55806-S2, front bumper; 55826-S7, 390 engine mounts.
The bottom line is there is little consistency even per car. All you need to do is to have clean hardware and it should not cost you judging points.
("Part number" deniers should not pay attention to "part numbers".)
Jim