theone,
Well, you could just use a Ford trans. If making motor mounts is no problem for you then making a trans mount for a Ford trans should be within your capabilities too. A C4 is a compact unit. A C6 is a nearly indestructible brute, but larger.
One of my 300 vehicles uses a C4, one uses a C6, one uses a toploader 4-speed, and three use GM powerglides. GM / Ford trans adapters are available. Your Skylark isn't like its sister Pontiac tempest with a rear transaxle and flexy driveshaft, is it? Go to the
www.fordsix.com website and all the major 300 dimensions are listed under the FAQ. Good luck with your project - it sounds as exciting as my buddies early 'Vette with a blown flathead Ford.