In all honesty, the best way to do it is just paint it up as an Imperial Guard tank fighting alongside your Marines. Anything that Space Marines would use a Superheavy tank for, Thunderhawks will do just as well if not better and they don't really have the transport equipment to haul baneblade type vehicles around. Fellblades are mostly a Bell of Lost Souls fan-fic invention anyway based off a 3rd party product picture of a Sons of Horus tank when Imperial Army troops still fought under Legion colors/iconography.
That said, if you just want to make it useable as a "it's a marine tank" just add tack on 75pts and make it BS4 for a quick derp-fix.
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