starting troubles

1 have a 74 duster 318. was having starting problems when it got hot. tried carb spacer helped for a little while, swap valve covers from chrome steel to cast aluminum 2 weeks ago to help with heat. this morning went to start it now it wont start. never had this trouble before always fired after sitting awhile. noticed fuel leaking from under carb spacer dont think thats is the problem. fuel getting to carb but not firing. turning over good. has a msd ignition setup. not very good with engine work so any help would be appreciated. thanks