Your batteries probably failed early because you charge them too fast and they over heated. The new Rolls will probably fail as well.
Lead acid batteries shouldn't be charged at more than 25% of their amp hour rating. Assuming that "four strings" means 4 batteries hooked in series/parallel, then you have 440 AHs. Both the engine alternator at 280 amps (wow, that is a beast!) and your two 70A Iotas will charge at more than 25%. If you go with 6 Lifelines then your charging rate gets to be within 25 % of their 660 AHs for the Iotas but not the engine alternator.
So I would assume that you have an external regulator on that beast of an engine alternator. Does it come with a temp monitor? Balmar regulators do, and will cut the charging rate in half when the temp exceeds their limit (I think about 120F).
With a temp sensor and 6 batteries I wouldn't spend the money on expensive Rolls batteries. Go with 6 standard GCs available at Costco, Sams, Batteries Plus, etc. and you should be fine.
BTW I think venting is entirely over rated particularly if you stay within these charging parameters.
ABYC requires a secure battery box that will catch the full contents of the battery. That doesn't mean the full size of the battery plus its contents, just the batttery.
No problem with big DC cables running alongside fuel tanks/lines.
Edit: Misread your proposed Rolls S2L16 batteries. I see they are 1169 AHs. That should work, with no temp sensor (barely) but I would add one anyway. Those batteries will cost a bunch!