Use 6-30 plug on 14-50 outlet
Chop the 6-30 plug off the heater and replace with a 14-50 (or 6-50) plug.
  




That's going to be the cheapest option.  Not bad if you know you're never going to move that heater around.  These plugs are readily available at home improvement stores.


  
  Get an adapter from 6-30 to 14-50 (or 6-50).
  


Flexible and reusable parts option.  Modeately more expensive, and those are not very common adapters to find.  You will probably have to build one from parts.  I'd buy a 14-50 stove code, and outdoor rated outlet box to put the 6-30 in.

Better yet, get an outletbox that's twice as large, and put a 6-30 and 14-50 on it, so you can keep both plugged in at all times.  Just keep in mind if you use both at once you're likely to trip the breaker just as if you turn on too many appliances in a kitchen at once.


  Are there better options (aside from running a separate circuit for the heater)?


You could run a subpanel.

If I was you, I'd run a 100A service from the main panel to a subpanel in the garage.  Garages are a great place for a subpanel.  If you decide you want to run both the charger and heater at once, you can, and you can add capacity for a compressor, or power tools later on easily, too.

I'd recommend if you add a subpanel in your garage to include a pair of 20A outlets nippled to the sides of it just to have it available.

You could then hardwire the charger and heater, and save cost and eliminate visual clutter of the outlets and plugs.


  Is there any advantage to getting the charger with a 6-50 rather than a 14-50? 


I would not expect there to be much difference, but you didn't say what chargers you're referring to.  I would buy whatever's cheapest, and I'd probably hardwire it to the subpanel to save on connector cost and risk of connectors breaking.


  Is 8/3 Romex sufficient?


I'm going to refer you to an NEC ampacity table so you can make that decision for yourself, but I wouldn't use 8AWG for 50Amps

Assuming you're using Copper and its going to be instaled in a wall, I'd use 6AWG.

Or run the subpanel with 1/0 (one ought) aluminum or 1AWG copper, and then I'd be comfortable using 8 AWG to a charger directly adjacent to the subpane.