Decking without removing the numbers can be done it is just going to cost you a lot more. I have never seen any one pop a hood at a car show and say oh look an IE application code! Only you will know that it is all original unless you make a big sign that states it is numbers matching. Even then only you an very few others at the show will care. All the show goers are interested in are those wide valve covers and what you have done to it to make an all original faster.
In 1966 the 427 was a brand new engine. The 427 in a Biscayne two door sedan was considered to be a sleeper; because no one knew anything about the combination. That, and expectations of performance were low compared to a Chrysler with a Hemi or a Ford with a 427 side oiler which where tearing up the track in that time period.
In 1966 the best performing car (that year's stock and super stock winner) was a 350 horse 327 in a Chevy II SS. (because it offered the greatest horsepower to weight of any vehicle this side of a motorcycle). In 1967 it was the 375 horse Camaro in stock and a 427 Camaro in Super Stock once again it was power to weight.
In 1968 the Impala and Caprice were running neck and neck in size and weight with a Cadillac that had a 472 cubic inch V8 powering it. Ford introduced the 427 SHOC engine that made Fords competitive again in racing and in stock and super stock they had the 428-429 powered Mustangs From 1970 through 1975 it was all Chrysler thanks to the 426 Hemi being offered in lighter car bodies.
Newton said it best with his second law of motion with F=ma. Nothing has changed since then, and it is all about power to weight which is why in the sixties it was all about larger engines being introduced every year. The 472 Caddy became a 500 cube engine, the BBC 396 became a 454, and though Chrysler lost the Hemi head to emissions, it still had their 440 as the big dawg.
You can not easily change your car's weight, but you can easily turn a 427 into a 496 and still have all the original numbers and decals on the air cleaner cover. Because the Mark IV block has the mains fed by an oil gallery it is limited in the length of the stroke, so the 4.25" stroke is the max for a Mark IV block. A Gen V block can contain a 4.5 inch stroke because the oil gallery was moved above the cam tunnel.
Using a new Gen V 454 block, that doesn't leak oil thanks to a one piece rear main oil seal, you change the displacement by lengthening the 4.25 inch bore block by adding to the stroke length.
427 had a 3.76" stroke
454 had a 4.00" stroke
496 has a 4.25" stroke
511 has a 4.38" stroke and a
525 has a 4.50" stroke
The 4.500 bore ZZ 502 Gen VI block adds even more cubes, and still looks like your 427. They all look alike from the outside. Retain stock mufflers and dual exhaust pipes you can speak softly and still carry a big stick to beat the competition with. The only 427 that I would run would be a stroked SBC 400 block thanks to today's gasoline.
Back when 104 octane rated Sunoco 260 was pump gas I ran an L-88 427 in my 1968 Camaro RS SS thanks to a Yenko built block in my Baldwin Motion III conversion. It put my Edelbrock tunnel ram equipped, Vertex mag fired, solid roller Crane cam Camaro into the low ten teens with 14 inch wide Firestone slicks to launch it. Power to weight!