Carb choice for a mutt motor.

A '68 318 with closed chamber early 273 heads shouldn't be low compression unless it's got a felpro 521SD (4.18 bore x usually .0625 minimum compressed thickness) head gasket or similar on it. Run a compression test for us. A well tuned 650 AVS2 is as close to perfect as you'll find for most nearly stock 318's.