Engine won't run

So here's the story.... Hopped in my daily driver (68 Cuda 74'318) started right up like always but then it backfired, I got out to chech under the hood ( while it was still running) opened the hood and it died. I started it again no backfire this time but only ran about 10 seconds before it died. I tried it again and got the same results, any sugestions? Ps engine has 2000 miles since rebulit and I've never had any problems