Wise Tail or not? : Running engine w/o exhaust manifold

[Twilightoptics]

I am almost done with my engine swap (my first and very... um complicated 305 TPI engine block to where my 283 was.) So anyways... before I bolt up the headers and such, what harm would come of starting the engine for maybe 20 seconds?

Secondly... how about running with open long headers? People say the block sucks up cold air.. well how far away from the block does that opening have to be before it wont cause harm? cars at the drag track don't have plugs or caps or have people shove rags in their open pipes that aren't very long when they turn the engine off? I ask this because I don't have the money to have new pipe welded up just yet.