I have the paperwork from royal motors on my 68 , was a 400 car. 428 was not a factory option ever

That doesn't mean it wasn't a dealer installed "dealer option" new with the car in 68? That did happen although usually at a customer request and not just to sell a car. If you have the original sales slip it might show that. My car has a trim piece down the side called "dealer installed deluxe trim". No a factory option but there it is.

"no arrow head emblem"
Also wrong color grills w/o center hockey sticks...

My dad had a used car lot from 1965-2002 and by the time I was 15 i had 15 different cars. And I loved hot rods. at 17 in 1980 I had a 1968 Firebird 428 4 B carb auto. It had a chrome breather top and valve covers AM/FM AC, power windows, air shocks, back seat would fold down for extra space, disc brakes. The rear end gear was real high it would would shift in to drive at 118 MPH when gas to the bottom. I totaled it when a old man in a galaxy 500 ran a stop sign. I was doing 75 MPH.

The car looked factory I just did not know how rare it was. My dad sold it as junk for $150.

Some of the cars I had. Not listing the normal every day drivers. And in no order.

68 ss Chevelle. 327 primer, black inter
69 Chevelle Red, 327 or 350 not sure Black inter
72 Chevelle 350 blue, black inter
68 firebird convertible 400 4 speed (fast) 72,000 miles. beige, black inter.
68 Firebird 428 Dark Green, Black Inter
70 Nova 307 beige Black inter
68 camaro 327 Black on Black
68 SS Camaro 327 burg with black strips, black inter
72 cutlass 350 rocket (fast) gold, white inter
67 mustang 289 baby blue, Black inter
69 mach1 fast back 351 Auto, White with red strips, black inter (fast). Did not have drive license yet.
70 grande mustang 302
70 camaro straight 6 blue
72 camaro 350 gold.
66 Chevelle white convertible. I still have a extra trunk lid for it. From 1984
57 2 door belair Black with a V8 dont recall the size but it was stock.
55 chevy truck short stepside Gold. 350, 350 turbo. I still have the all copper stock radiator.
82 firebird T tops gold, black inter. v6. Put in a 305 HO and trans, computer from a 86 IROC Z.
71 Z28 camaro, 350 LS1 Blue, black inter, custom paint uni-peg and moons with "Born To Run" on the side with side pipes chrome wire wheels. The fastest of all the cars.
79 Z28 I still have it with the motor from the 71 above in it. Brown with brown inter.
72 cutlass S 350 rocket 350 turbo blue with a white top, white inter. 8 track tape made in dash. 92,000 miles factory paper work Still have, last car I ever bought from my dad.
85 chevy S10, 350 double rolling chain, 490 lift with AC

I am sure I missed something. I have seen a lot of odd cars.
what about a chevy van with a 4 speed on the column?
A 68 camaro with a bench seat?

No Firebirds came from the factory with a 428

Welcome Olin. Wow you have a good memory and what a great list of cars. With a 428, a/c, power windows and fold down seat it sounds like that Bird flew in style!

If he has a paper trail it is possible, here's why and only why.

First off NO Firebirds came from the factory with 428's PERIOD.

Second - a very few select dealers would put one in for you, one I know for sure did this was Myrtle Motors in New York. I believe Royal did too.

They would have the original paperwork from the dealer and the paperwork better look real old, no copies, it's the only way to prove it. PHS will have no history of this being done but if the papers exist it would be a rare and valuable bird.

A lot of people did this at home too, the 428 is a plug in.