Yes, the tire manufacturer's recommended maximum pressure always presumes hook-bead rims these days.
But there are a lot of factors involved with un-easy or incompatible fits between tires and rims. I wonder whether even the size of the tube you use could make a significant difference?

Have you considered the inside width of the rim walls? Not all rims are made equal. Some earlier rims were actually even stamped as "27 x 1-1/4" or "27 x 1-1/8" - and this was for a reason. Some narrower tires are just not suited to certain wider rims. And now there is an even greater difference in actual tire design too. Most older 27" tires were simply shaped differently in cross-section than modern 27" tires of the same nominal width - probably because the side walls were thicker, stiffer.

For example, I love my old Super Champion model 58 rims which have flat walls and no bumps or bead hooks at all. However, I always use wider tires with those. That was in an era when virtually any race was ALWAYS simply ridden on sew-up tires, and clinchers were typically just heavy gum-wall tires suited for leisurely recreational riding. A typical older clincher tire would weigh well over a pound (454 grams). And there was no such thing as a folding clincher tire for MANY years to come.

The Mavic module-E (designed specifically for use with Michelin's new "Elan" clincher tires) came out around 1975 and had a distinct hook, because they were presumed to be used with those lighter weight, thinner sidewall, and narrower width clincher tires more suited to fast riding or possibly even [gasp! ] amateur racing. So, those were really perhaps the first truly modern style rims I can think of and were the start of an eventual revolution in the practicality of lightweight clincher tires which now are entirely suitable for racing.

Most 27" tires of the 70s were not only wider, they seemed "taller" they had more of a "U" shaped profile compared to the rather "round " cross section of the lighter weight modern Performance GT2 or GT2K tires. If I use any of the Performance tires, I definitely use more narrow hook-bead rims. And, the basic old tires also had thick gum coated sidewalls too... just think of the cheapest Kenda or Cheng Shin $7 tires you can find today, and this might be close to a standard better quality Michelin "Sports" tire of the 70s. Sorry, but most earlier 27" tires were just not at all high tech, and most rims were really made for these real beefy recreational tires. Narrower and thinner walled modern tires require narrower rims for best results. I believe Sheldon Brown has a good extensive article, complete with charts derived from one tire companies experiments and recommendations for tire and rim width and tire pressure compatibilities.

Now, on the other hand... you can safely go pretty crazy with hook-bead rims and have no problem loosing a theoretically incompatibly fat tire. On most modern tires the sidewalls are very thin and so they can still fit in the narrow width modern rims without any difficulty. You could never do this with "Old-style" gum walls because the beads would probably be touching together while you were still trying to finesse the deflated tire into the narrow rim cavity.

But, before you give up on the rims, look for the recent thread where someone had trouble fitting a tire which seemed too tight to pull onto his rim. Difficult fitting tires, in this case, might be a GOOD option for those rims.

Modern big fat-assed city/hybrid tire 40+mm wide, but with thin side walls.
Looks Loony, but works fine on these narrow hook-bead Mavic Open Pro rims



Old high quality Michelin "Sports" tire from the early 1970s. 525 grams of 27" durability
... and please, no nonsense talk about "Racing" in their presence!