The film back you are using absolutely matters, because this whole concept of "equivalence" is in reference to a given film or sensor size.

You have a 3¼×2¼" mask, so that's the size of your recording medium — about 82×57mm. We're comparing that to the sensor size of a "full-frame" reference DSLR, with a sensor the size of 35mm film — which is 36×24mm.

You'll notice that these aren't quite exactly the same aspect ratio: your mask is a little more square. In this situation, we usually compare the diagonals, which are 99.9mm (let's just say 100!) and 43.3mm, respectively. This 43.3mm/100mm gives us 0.43× as the "crop factor". (That is a strange term sense because we're taking it in the opposite direction of cropping, but I'm going to keep using it anyway because it is the typical term and is even used in this way for digital medium format.)

If the two aspect ratios were really far off — like, if you had a 2¼×2¾" mask — you might need to crop off even more in order to get the formats to match in shape, you might want to calculate that separately. (Like I did here.) But here, we're basically within all reasonable tolerances — so 0.43× is our magic number.

5½" is 140mm, so the 35mm-equivalent focal length of this setup would be simply 140mm × 0.43, which is about 60mm. In other words, you have a slightly-long normal, and a little short for what would normally be used for portraits.

This equivalence holds for angle of view and also holds for depth of field, given some assumptions, like equivalent print size. That is, if you shoot wide open at f/4.5, your depth of field is roughly the same as if you were shooting with an f/1.9 lens on a full-frame DSLR. (This doesn't affect exposure calculation, though!)

If, instead of the 120 back, you were using a Grafmatic Sheet Film Holder with 4x5" film, the frame area would be just a tiny bit smaller than 4×5" — about 96×120mm. If you cropped your DSLR image to the same aspect ratio, you'd have a crop factor of 0.3×. In this case, your 5½" lens's field of view would be equivalent to a 42mm lens on the DSLR — still in the "normal" range, but at the wide side rather than the long one.