BOOM! Wont start, severe backfireing

My friend has a 79 350 chateau van. He sais it's been deiseling recently after turning the engine off. Yesterday he drove it to a restaurant and when he went to start it up again it wont run. He pumped the pedal acouple of times and tried starting it again and BOOOOOM! it backfired and scared the shite out everyone within a blocks' radius. It sounds like a freakin cannon!

We've tried to pour gas in the carb to see if it had run out of gas, it didn't start.

We looked at a couple of the plugs to see if they were fouled, they look fine.

It wasn't like an ordinary POP backfire, so we're concerned about starting it again without some advice. I'm sure the restaurant would appreciate us moving the van a soon as possible. Thanks for any suggestions.

replace dist cap and rotor for sure, plug wires too if you have time

Thanks for the advice, we'll give it a shot.