The reason it costs more is almost everybody has eights, Heck you can buy billet 8 junk at 2a.m. by phone.
BUT
Inliners are WAY cool. If you're interested in hopping up a sixer and want info on head work, intake and carburation ideas, split exhaust, rockers, HEI, and more ...
The point here is what I remember from my old high school days, when you change one thing that starts the dominos falling and pretty soon that small project got bigger.
check out:
http://www.customdesignperformance.com/ they advertise in 'inliners' also
http://www.customdesignperformance.com/leo/leo.html and cliffords (as mentioned above) as well as this site:
http://www.truckworld.com/How-To-Tech/Chevrolet-Engine/Chevy-engine.html Buffalo Enterprises Contact Steve @ (360)652-7684. an old knowledgable sixer and great guy.
join:
http://inliners.org/ ,
http://www.stoveboltengineco.com/ talk to Tom.
As far as putting an L~6 into stovers, the motor mounts need changing, 700R4 or a T-5 w/ all camaro guts are wonderful backers to it, what ever pleases you. 420's and 465's are very strong but not what you want here if this is your goal. You can pump out tons of HP out of an L~6 but then the other factors come into play. Change out the straight front axle, or at least change kingpins, install disc brakes and a dual master. An IFS??. You have already changed out the rear end and probably have good brakes back there. But I have to say I'm looking forward to my sleeper 51 bolt. Buy Leo's book, it's a good start and may save you some headaches, but many of us wish he had put more in about streetable 6er's. Read as much as you can about your new project. Good luck having fun w/ her.