Probably the easiest is to use the '68-'71 engine mount brackets and mounts. In fact everything for the '68-'71 will work in a '63-'66. You may need to trim the top flange of the frame for manifold/header clearance. Here is a thread for some more info.
http://www.67-72chevytrucks.com/vboa...d.php?t=551120
When I installed the 454 on my '66 Suburban in 1992, I used the '73+ clamshells and mounts but made my own brackets to accept the mounts. That way I got the engine exactly where I wanted it. Close to the firewall without having to modify the firewall.