How do I recalibrate this encounter from Red Hand of Doom for 5th Edition D&D?

I’m running D&D3.5e’s Red Hand of Doom updated for 5e. So far it has been effortlessly straightforward. I’ve been keeping the basic combat encounter structures and simply swapping out 3.5e monsters for similarly-named 5e monsters (3.5e hobgoblins for 5e hobgoblins, 3.5e manticores for 5e manticores, etc.). Likewise, I’ve been winging skill checks and other non-combat challenges just by eyeballing how hard the stated 3.5e DCs likely would’ve been and using my best judgment to apply 5e DCs of roughly similar probability given bounded accuracy.

However, the PCs are coming up on a critical encounter with a major non-combat objective that presents special conversion problems. The encounter in question, arising near the end of Part I, is

Because this objective isn’t a monster, I can’t simply turn to stock 5e monsters and assume all the calibration will have been done for me. At the same time, it’s not as simple as a skill check that I can just eyeball. RHoD provides 3.5e combat statistics for the objective (see p. 34-35), but I’m not sure how those statistics translate to 5e. As written, the objective has what I perceive to be an outsize pile of HP, plus additional defensive features (taking reduced damage from certain sources, etc.) arising from how 3.5e treated entities in the nature of this objective. It’s not clear whether, or how, 5e might expect me to recalibrate those statistics and features.

That is problem enough, but RHoD also goes out of its way to enumerate 3.5e spells that can interact effectively with the objective — most of which are either unavailable or fundamentally changed in 5e. To wit:

  • Soften earth and stone does not exist in 5e.
  • 5e’s version of stone shape restricts the effected area to "no more than 5 feet in any direction," which was not a limitation of the 3.5e version.
  • Stone to flesh does not exist in 5e.
  • Transmute rock does exist in 5e and is substantially more useful in that it applies to any nonmagical rock, rather than only natural, unworked rock as did the 3.5e version. The 5e version could probably deal with the objective in a single turn, whereas RHoD says the 3.5e version just dealt some modest damage if used in a particular way.

Given the different combat mechanics and spell functions between 3.5e and 5e, how do I convert this encounter so it remains an appropriate challenge?

(In case context is helpful, the party is level 5 and comprises a Light cleric, a Hunter ranger, a melee-heavy Battlemaster fighter, and an Abjuration wizard. Despite RHoD being written for 3.5e parties starting at level 6, up to this point these 5th-level PCs have been able to handle the encounters in Part I of the adventure.)