To answer the OP, no, they don't need one.
The Giants utilize the "four aces" type package on obvious passing downs anyway. If they had an elite pass-rushing DT, sure, they could add him to that package, but there are only a handful of those guys in the league at any time.
Hankins plays the under technique against the run fantastic. Everyone is talking like that is just a pass rush position. We're so spoiled by the impenetrable run defense they had last year that we're just taking it for granted. It was the combination of Harrison and Hankins that made that work. Hankins had seven tackles for a loss. Just watch film of any game and you'll see it was not Harrison that was most often doubled, it was Hankins. Hankins played about 10% more snaps than Snacks. He swims under the guard's inside shoulder and forces the center to come over and chip him, leaving Harrison freed to slide down the line and make tackles. You can watch tape of the Dallas game, for example. You'll see Zach Martin tapping Tyler Frederick on the hip over-and-over, asking for help with Hankins whenever he shades inside of him.
The fronts Spags used last year are a hybrid of the old Jim Johnson schemes he came up with and the unique fronts the Ravens use where guys play hybrid techniques. So the equivalent you might be familiar with would be Haloti Ngata, who can play multiple techniques effectively. He doesn't put up huge sack numbers, but his ability to penetrate/eat double teams is critical to their effectiveness against the run. Ngata is 6-4/350, he's got a Hankins-type body.