So I know that you can use a guitar through a bass amp, but if say I wanted distortion, could I, and how could I do it? I know Doyle from the MIsifts uses a bass head but I'm not sure how he got distortion. The level I want is for metal and punk rock. Suggestions?
Yes you definitely could!

If the amp itself doesnt have proper distortion (they mostly dont) you need a pedal for that. I would recommend a distortion pedal (ie. Boss DS-1), not an overdrive pedal, especially if you want to play heavier metal styles.

Only fault i could say about this setup is that bass amps focus on lower frequencies (obviously) so you get really bassy sound, but its all about your taste!
Metal is like a apple, no-one likes the core.
It depends on the bass amp, of course. But most bass amps these days are full-range rather than bass-centric (thanks to slappers, pickers and 6-string players), and most bass cabinets lean toward full-range as well (unless you're concentrating on dubstep, etc.). You may find that you have MORE range than you do with a standard guitar amp, and you should have a surprise waiting for you at the bottom end of the fretboard.

Treat the amp as if it were a clean amp, don't bother trying to overdrive it, and produce whatever distortion you want via pedals rather than trying to make the amp produce it.
GessuThanks for the suggestion! I have an Electro-Harmonix Metal Muff pedal so I could do that! Thanks alot!
Really depends on your speakers as far as I am aware, think of it like a car stereo system if you have big 14 inch low frequency subs, theres no way they can pull of the high or shrill notes, but if you have a full range speaker like what you find in your off the lot car, then they will be more than able to tackle your guitar.

im not too sure about headers as ive never ventured into the world of tube bass amps.
It can work. Doom metal frequently uses bass stacks on guitar. But the speakers are important. If it has a tweeter in it like my little baseman 60, it will sound shrill and gross. Great for old-school black metal. But that's it haha.