How much is your budget? Remember, for dial type/mechanical units you need the desired temp to be in the mid range of your thermometer. Just a starting point to consider. I have a non contact that works good but then you also need to think of batteries with it.
Lead, Pewter, Zinc, Bullet Casting Thermometer 6" Long same as RCBS: Stainless Steel Metal Raw Materials: Amazon.com: Industrial & Scientific Amazon.com: BAFX Products Non Contact Infrared (IR) Thermometer Range of -58F to 1,022F Includes Battery: Industrial & Scientific Pro and Con. Electronic may be your wife's favorite. Mechanical will always be yours and easy to find due to lead poisoning possibility, of course you can always leave it in the pot to cool!.
Go mechanical like HK said, The IR will simply reflect off the molten lead and not give you a true temperature, Unless you set a pure copper penny on top of the lead and shoot it with the IR, Go mechanical.
Looks like a typical "Plumbers Pot". Sewer pipes, cast iron as well as ceramic were sealed with tarred jute and then hot lead. Some homes had soft lead kitchen and bathroom drain pipes that were soldered together using this heater or a blow torch as a heat source for the soldering iron. Vintage Plumbers Furnace Burner UNIIQUE MFG.Co No 50 Gasoline Melting Pot was used to heat the lead with that style pot
http://images1.americanlisted.com/nlarge/vintage-c-l-plumbers-fettle-with-pot-and-spoon-americanlisted_35442355.jpg I looked for images of vintage plumber's pots and that one looks spot on.
Man, I would never attempt gasoline to fire a furnace/burner, Simply because I've worked with Nat. Gas for several years. Those guys had some Big' En's.
For Real ! Hey Kellory, I use the bottom spoon for removing the dross from the top and also stirring the pot, Locally this is know as the Groout, You can make your own with a grinder.