Sorry. But I have to disagree on that score, @Xulfiqar. Were this a Bugatti, or a Rolls, or a Ferrari (whose value and novelty actually hinges upon its originality and rarity), I'd strongly urge the owner to "stick to stock". However, this is not one of those cars.
These affairs become a matter of individual preference (notably, the owner's), availability of parts, logistic 'doability' and technical feasibility of the rebuild team. Something which is bothersome or worrisome or irritating, can (and should, in my opinion) be swapped with something better, which provides ease of use and peace of mind.
I have a '67 Impala which I lovingly rebuilt to stock (at considerable cost), and it drove like a horse-drawn funeral hearse. Like an absolute dog. Pedestrian, noisy, temperamental, and a complete and utter pain in the behind. And no...contrary to popular belief, a temperamental car is NOT a quaint, adorable, quirky feature to admire. The car is now getting a Toyota transplant (1UZ-FE VVTi), and I'm certain that I'll be very happy with it, indeed...since I plan to rack up mile after mile after mile in it. Were it to sit pretty in a glass cage, and be used on 14th August every year only, on the other hand, the stock setup might've been just fine. I WILL keep the 350 small block V8 in storage, should I choose to go the "stock" route again...but I very much doubt I'm ever going to use it. Perhaps my son might, if he has better tech skills, better access to parts, and plans on making the Impala a museum piece at any point in time.
@VEEeight: You're absolutely right about the 1UZ (even the 3rd Gen) giving more power (260 bhp over 247 bhp) than the 6.3L V8 that powered the 300SEL 6.3 and made it the fastest production sedan in the world for very many years. This is very much Q-Car territory. Personally, I think it would be a beautiful sight (and sound) to watch a big bruiser like your Merc, take off into warp-drive from the traffic lights!
Just my two cents. Thumbs up, whichever way you decide to go.