You will have to talk to the rescue whether they are are OK with that, as it is just below RSPCA recommendations. Would it be possible to switch to a 2x4 C&C grid pen, which is roughly the same ground space as a 2x5 ft cage (compliant with RSCPA minimum recommendations), but as it is 70cm deep, it is not longer?
Boars are generally old enough for neutering around 4-6 months of age and then should have a 6 weeks post op wait to make sure that they 100% safe to go with sows - especially adult ones at higher risk of birthing complications. I have the daughter of a supposedly safe over 5 weeks post neutering op rescue boar, so it can happen as late as that and have since heard of other cases.
If your girls can date neutered boars at the rescue for character compatibility, then that would be great. Not every sow and boar will gel, especially if both are dominant. I had to jiggle my groups carefully so they have either a dominant husboar or a dominant top sow at the helm.
I currently have 4 boars living with 2-6 sows each, and they are very happy boys indeed! But it would pay to do your research first!
Good Luck