Need help with rebuild

We have rebuilt a slant six 225 for a 1964 Dart. We're done with the rebuild but can't get it to start. It'll rumble like it's going to start but won't maintain and stay running. We have spark, timing is set (and double and triple checked), distributor is new, starter works well. The guy that helped me with the rebuild is very good at working on cars and has rebuilt many engines but this is his first slant six.

We're at a loss as to what could be preventing this engine from running. What else could we be missing? What are we missing?

Thanks.