Modifying a Rare Classic Car Can Wreck Its Value. Modifying a Common One Can Raise It
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5 min read
A numbers-matching 1970 Plymouth Hemi 'Cuda convertible sold for $2.53 million at Mecum's Monterey auction on 17 August 2024, a record for the model. A little over two years later, a 1970 Hemi 'Cuda hardtop from the same model year, its factory engine swapped out for a custom-built replacement, was still taking bids at just over $78,000 on Hagerty Marketplace in mid-September 2026. Same base car. Wildly different outcomes. Classic car modification can gut a collector car's value, or barely touch it, and the difference comes down to one question: is this car rare enough that buyers are paying for originality, or common enough that they're paying for a fun car to drive?
Collectors use the term "numbers matching" for a car whose engine, transmission and rear axle all still carry the serial numbers the factory originally stamped on them. It is the strongest proof a car exists exactly as it left the assembly line, and it is the single biggest lever on a rare classic's price, more well documented on paper than any restoration receipt. NetWort's own piece on classic car appreciation already established the pattern: a car needs real rarity, documented originality and an active collector market before its price trend flips from falling to rising. Modification is the one thing that can undo the "documented originality" leg of that stool on its own, no matter how good the rest of the car looks.
Plymouth built only 666 Hemi 'Cudas for the 1970 model year, just 1.3% of the nearly 49,000 Barracudas it sold that year, and only 14 of those 666 were convertibles. That convertible rarity, combined with an intact original engine and drivetrain, is most of what drove the $2.53 million Monterey sale. Classic.com's own sale-price data for Hemi 'Cudas over the past decade puts the average across the model at just over $300,000, itself already a rarefied number for a 55-year-old car.
Against that backdrop, a modified example sitting at $78,000 in current bidding, well under even that $300,000 average, makes the point plainly. That specific car had been in one family for 30 years and had just been treated to a fresh respray and a rebuilt drivetrain, care that would normally support a strong price. It is a hardtop, not a convertible, so some of the gap against the $2.53 million car is body style and rarity, not modification alone. But it is still the same base model, the same era and the same badge, sitting at roughly a quarter of the broader model's own average, specifically because its original 426 Hemi is gone.
| Car | Status | Price |
|---|---|---|
| 1970 Hemi 'Cuda convertible | Numbers-matching, 1 of 14 built | $2.53 million (Mecum, Aug 2024) |
| 1970 Hemi 'Cuda (all variants) | Mixed, decade average | ~$300,000 (Classic.com) |
| 1970 Hemi 'Cuda hardtop | Modified, engine replaced | ~$78,000 (bidding, Sep 2026) |
The same logic runs in reverse for common muscle cars. Hagerty's own market coverage has repeatedly pointed out that a well-built restomod, a classic body shell fitted with a modern engine, suspension and brakes, can now sell for as much as or more than a numbers-matching stock example of the same high-production model. Barrett-Jackson and Mecum both run dedicated restomod categories today, and a heavily modified first-generation Camaro or Ford Bronco regularly crosses the block for real money: a modified 1968 Ford Bronco sold for $115,500 at Mecum's Nashville sale in 2026. Nobody is paying that for a Bronco's numbers-matching originality. They are paying for a comfortable, reliable, good-looking car they can actually drive every weekend, and a modern drivetrain adds to that, it doesn't subtract from it.
Ask the same three questions NetWort's classic car piece already uses for appreciation, because they also predict how modification will hit you:
If your car answers yes to the first two, keep it original. Reversible upgrades only, if any at all. If it does not, and you were always planning to drive it rather than show it, a thoughtful modification is far less risky to its resale value than the "in half" reputation suggests, and can sometimes help.
If you already own a classic car, whatever category it falls into, NetWort's Holdings page lets you log it under the Vehicle category and set your own expected annual rate instead of the default -10% a year, so your portfolio reflects what you actually believe about your specific car rather than a generic depreciation curve. Before you touch the engine bay, check where your car sits on the two questions above, and if you are still unsure, our piece on vintage cameras as a collectible asset walks through the same rarity-and-documentation test for a completely different kind of collector market.