I posted about this several months ago. I purchased one just like my friend has in his '31 A hi-boy. 40,000BTU on high drawing 40 amps. I got this thing hooked up and installed on the Speedster...Let me tell you it is a primo investment...Today it is 40* out, I have the tonneau on, drivers side down. Left the house and fired up the heater on high and high blower after about a block...Three blocks later I'm turning the heat setting down and the blower to low. Drove it for about 20 min. this way then came home as it was starting to rain, stayed nice and warm on the lower half of the ol' bod. If the top was up it'd probably be toasty warm
What I used: Have it mounted in the trunk area with two air hoses going into the cockpit at the firewall. Not cheap about $250 shipped but well worth it if you want to stay warm. And the 40amp draw is only when the blower and heater are on high settings so most 60amp+ alternators should keep up with it. At low blower and temp settings it only draws 15-20 amps...