The biggest misconception about fishing with Spey lines (however long the belly is) is that you 'have' to cast the whole head length [to get the 'best' out of the rod].
For those of us dinosaurs who learnt how to fly fish before the short shooting heads & certainly the ultra-short Skagit heads were ever conceived, we started with 105 to 120' double taper lines which actually don't have a "head" length, and we just learned to cast with sufficient of the line out of the tip-top ring which we felt we could manage (and depending on the river width), and shot the rest of the line as needed. Funny enough, those rods didn't actually 'demand' a certain specified length of line out in order to load the rod for casting - if you needed to cast 10 yards, fine; if it was 20 yards, also fine, if it was 30 yards, still fine......you just had to alter the timing of the D loop formation depending on the amount of line you were loading the rod with.
It was only some decades later that most fly lines became weight-forward, which then brought in the concept of the bulk of weight of the fly line being in the weight forward 'head' section, the remainder of the line being the running line.
The advantage of the weight-forward concept is that the caster can use a more manageable length of line out of the tip top ring, and shoot the rest of the line as needed for distance, and casting is more 'efficient' (aka easier).
Most of the modern short-belly, mid-belly and even long-belly 'Spey' lines have head lengths under 95'; competition casting 'Spey' lines may have head lengths to 105' or more.
With the concept of concentrating most of the casting weight of the fly line in a shorter section of the line (ie weight-forward) already established, other innovators (Scandinavian & others) played around to develop even shorter heads (Scandi heads), and even later to the game came the ultra-short Skagit heads (originally just ~27'). With each development and innovation, it was discovered that shorter heads are indeed easier & easier to cast, bringing a substantial boost to the numbers of fishermen who could get out on the water and be fly fishing with a short learning curve, in a short period of time.
Going back to the OP, getting into using a mid-belly 'Spey' line from the shorter head systems is fairly straight-forward, and probably the Airflo Delta is as good a place to start as any; just start out with casting less than the full head, get the feel, practice & more practice will allow you to cast more of the head, most of the head, all of the head, and even shoot some line. Baby steps......it's NOT rocket science, nor should it be....
Some of us dinosaurs still use double taper lines (sometimes)
Good fortune with your continued journey....
Mike