I think you should go for it. An L18 will overbore just as much as an A15, so 1900 cc is available. Having driven a Sunny with a big HP A15 & another with a stock L18 out of a ute, the difference in lap times wasn't that much, & if I'd read that L series book I'd have just grabbed a cam from one of the L20B's & put it in at timing mark 2 or something like that. The engine would have then revved to about 7000 rpm, instead of dropping dead at 5300. You might have to change your diff ratio, I went from a 4.6 to a 4.1 diff ratio, but the car would still pull around 190k's (120 mph) with the taller diff. If somebody you know has an L18 sitting around doing nothing, I suggest you borrow it for a season. Is there any reason you don't go straight to an L20B? There's little difference in weight & I know they fit into 1200's just as easily. So long story short, go for the L series.