Don't want to stear you away from this forum which is great, but there is a very good 55-57 one called http://www.chevytalk.com
As stated you can put all the small blocks in without a problem. You can even stuff big blocks in there. Just have to change the motor mounts and massage the firewall usaully. As far as purchasing an engine I would deal straight thru GM. They sell crate engines with good warranties.

Brian