Just go ca and pocket the change then. Stock ca will get you close to a 12 if you sort the handeling.
Your also going to need stumpy gear box and a strong rear diff set up. More weight.

For that level of performance and power that extra 10kg if l series vs l z head vs z18et is going to mean pretty much nothing in terms of dulling performance.

If anything I find my 1200 to road hold better with a bit more weight in it for road driving- even in fun.

Stay clear LPG if you are like me and don't want to develop it properly- my system compromises outright performance and is carb like. Stus is leaps and bounds ahead. And there's still vapour or LPG injection but out if my budget lol.

Plenty of boosted l series 1600 and blueys out there, old school technology will get you almost your goals cheaply reliably if kept simple