There are 5 liter Miata conversions flating around as well, but word gets around that a forced induction engine built (forged lower compression pistons, con rods, etc.) and turbo or supercharger will produce similar power at lower cost. It's not like doing an engine swap on a 60's body on frame american car where a little tinkering and a blow torch can solve most issues. Unless you're building a dedicated trip or road track car, it's probably not a realistic project. $20k can get you a nice SRT6 coupe which can easily do 325-350 wheel hp with only a CAI, pulley and tune ($~ $2,500). A stock LS1 will get you 290 at the wheel, and the conversion would cost about the price of the SRT6.