Hi Sixnut - if you are keeping the front struts, cutting coils off the spring to lower the stance will increase the spring stiffness, so the front will be lower, but you'll be skipping all over the place. You definately need softer springs, which should compress more and lower the stance that way.
Is there enough room under the bonnet to raise the strut top mounts? That would lower the ride height without changing the spring rate. You'd still need softer springs though, you've taken out an engine and transmission + driveshafts and you are putting only a fuel tank in their place. What would that equate to? A 300 lb reduction? Split between the two front wheels 150 lb per wheel, same as the spring rate, so at least 1" higher at the front.
To do the job properly you need to know the corner weights, and measure the unsprung weight of the strut/wheel combination, set a target suspension natural frequency (cruiser/sporty/full race/GFX) and work out the resulting spring rate. As the natural frequency at the front needs to be lower than the rear to stop front/rear pitching effects, for your mid engined machine the front spring rates will need to be quite a lot softer than the rears (which are 164 struts, what is the spring rate?). You won't find any Alfa springs at rates of around 100-120 lb/in (which I think you would need), I would check out manufacturers who can wind a custom spring, or as Kev suggests, maybe the front struts from an X1-9 would do the job (they'll be shorter too!).
Conversely, now that most of the cars weight is on the back wheels, you might find that the 164 springs are too soft. There again, the Sprint shell is probably lighter than the 164 bodywork, but my point is you will need some fine tuning or the car will be a nightmare to drive (and I've got three complete sets of redundant springs in the garage to prove it!)
Green Machine has a custom fabricated alloy fuel tank. Its TIG welded and had a number of pinhole leaks which had to be painstakingly rectified (not by me, my welding is of the "pidgeon poo" variety
) with the tank removed from the car and drained out. Fuel tanks can be fabricated out of steel as well, which might be easier to do.
With plenty of space under the bonnet, I would go for a proprietary racing tank, one of those alloy ones or if you are feeling flush, an ATL fuel cell.
Still making excellent progress. Nice beading on your engine bulkhead by the way