I also have a 521 i just bought, the guy i bought it from also gave me a dogleg and an l20 block that needs a little work, he says that the dogleg will only fit on the l20, but i keep hearing people say it should mount any L series. I'm not sure if he said there was something different with the fly wheel or what, and since you have completed the swap i was wondering if you could let me know. Other then that, I heard the only modification is the transmission mount or crossmember, if you could let me know about that also, that'd be great. Thanks, Lerin