If the ASA can seperate from the drop, the easiest (and probably the cheapest) is to just get a rail to put between them. You can probably find a used one for ~$10 shipped.
Another option would be to get one of those reg extenders from MacDev. However, this option kind of defeats the purpose of the drop since it would be moving the center of mass toward the back end of the set-up. The extender is basically just a piece that would go between the tank reg and the ASA.
As for getting a larger drop, there are larger ones out there. First one that comes to mind is one from Psycho Ballistics. This one: Psycho Ballistics bullet drop forward.html
I have one on my old Spyder. I have modified it a tad since the following pic was taken - I counter-sunk the screws holding the drop to the gripframe. This gave me a little more room. Picture of set-up for size reference:
Graphite Dangerous Power REV-I
– TechT bolt engine, REV-I & Disruptive barrel kits, Prophecy V2 w/ Empire feedgate, Crossfire SS 68/4500
Maroon and Polished Aluminum Dangerous Power Threshold
– 7th Element board, TechT bolt engine, magnet trigger kit, REV-I PMD, Sly Dual Carbon barrel kit, Prophecy V2, Crossfire SS 68/4500
Black and Silver Dangerous Power F8
– APE Rampage board, VS bolt, magnet trigger kit, F8 gauge, REDZ barrel kit, Halo B w/ Exalt feedgate, PMI 68/4500
Polished Aluminum and Red Dangerous Power F8
– Virtue board, VS bolt, magnet trigger kit, Halo B w/ Exalt feedgate, PMI 68/4500