Wiring – reuse hot tub wiring for an EVSE

1. 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.

2. 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.

Temperature at the terminals is handled for you

The NEC's ampacity charts handle temperature at the terminals for you, simply by way of you selecting the appropriate column from the chart for the temperature limit of your terminals.

3% for feeders is a good voltage drop rule of thumb

The reason 3% is used for feeder voltage drop is because we want overall voltage drop to be no more than 5% at max ampacity (some stuff can take more, but it's not particularly kind to certain loads, esp. motors), and the other 2% needs to be left for voltage drop across the branch circuit to the load in question. Varying this is possible, depending on the load, though -- a water heater might be OK with a bit of extra voltage drop, while your air conditioner would be better off with the full 240V at its terminals.

Of course, if you wish to analyze load diversity and come up with a more realistic design point than maximum load for the feeder to be at design voltage drop, that's your prerogative -- as long as the wire meets Code minima, it'll be able to handle the current safely.

Will it fit?

There are two final things you need to check:

1. Is your conduit big enough for your wires? An overstuffed conduit may overheat, in addition to making the pull far harder than it needs to be -- in fact, it's wise to oversize the conduit to keep pulling difficulty down and provide room for future expansion.

2. Will your wires fit in the breaker lugs? Most lugs accept a wide variety of wire, but it's good to double-check before you have everything pulled.

P.S. TORQUE MATTERS

Section 110.14(D) in the 2017 NEC requires that connections be torqued to manufacturer specifications; in practice, you'll need a torque screwdriver and/or torque wrench, both reading in inch-pounds, for this.