I'm not a fan of invasive, irreversible mods. They aren't necessarily wrong, just potentially regrettable. A lot of guys did a lot of mods to old Fenders and Marshalls back in the 80's... things that probably seemed like a good idea at the time but now they just make you shake your head. Once you cut a hole in that chassis you can never put it back.
So for me personally, I'd just replace that cord with a new one. Get a nice heavy duty cord and a new strain relief and that amp will be good for another thirty years. I don't see any great advantage in a removable cord anyway. I've owned heads with fixed cords and detachable cords and each has advantages and disadvantages. It's "six of one; half dozen of the other" as far as I'm concerned.
One other thing to consider: A new power cord and strain relief is a quick straightforward job for your tech. Drilling and installing an IEC connector will take considerably longer and, hence, be more expensive.
Anyway, your amp; your choice. Do what you want. I'm a part time amp builder and sometime amp tech and I just wanted to weigh in with my perspective. After many years of poking around in old amp chassis, I can't think of any invasive mod I've seen where I've said "yeah, the amp is better because of that."