Tom,
Here's the deal- you want heat? Good luck. A pan based car leaks air from a lot of different places- under the dash, between the back of the pan and the body behind the front seats, up the tunnel, in the soft-top around the side curtains- everywhere. Heater boxes are great in theory, except that they rely on the engine cooling fan to drive heat into the cabin, and they are supplied with 100% outdoor (engine compartment) air. The engine has to be fully warmed up to make much heat, and if your engine doesn't have a thermostat (and most don't), forget about warming up unless the temperature is over 20 deg outside.
Gas heat would be great- if you are building a car new I would definitely do it. If you aren't, I'd forget the idea. I've even explored the idea of buying a Webasto pre-heater (it's like a little gas or diesel fired boiler) and fabricating a coolant system to feed a heater coil in the cab (don't bother- it would cost over $2500 to retrofit). I've thought of installing an oil cooler in the engine compartment and feeding the discharge air into the heating system ductwork. Recirculating cabin air through the heater boxes would be great, but would require considerable lengths of ductwork, all outside, and all subject to cooling the inlet to the heater boxes.
The first order of business is sealing air leaks- you could have a camp-fire in the passenger seat next to you, and if you have air whistling through the cabin, you're still not going to be warm. I went to a considerable amount of expense to have a hardtop built for my car- it leaks as badly as the soft-top, and cannot be easily modified to be made water/ air tight.
The point here is the same one I had alluded to initially- if you only knew what most long term owners in colder climates have gone through to get an extra couple of weeks out of these cars, you would be stunned. If you live in a cold climate, no matter what you do, you are not going to have good heat in the sense that you are used to in any car built since 1960 (with the exception of air-cooled VWs), unless you go to considerable time and expense.
All this has been covered many times in the forum. The suggestion to search for heat related posts WAS informative, and meant to be helpful. My advise- either pay Henry at IM $35K for a nice roadster with gas heat- or just figure on putting the car up in November, and getting it out in April. I wish I had all the money I spent trying other alternatives back- I'd buy a nice 2110 from Pat Downs and take my family on a vacation with the difference.
