Back in 1999 when I 1st set up my '74 one of the 1st things I did was have a top w/4 sided enclosure made...wasn't a matter of adding winter to the season...we were fishing 12 months anyway...was a matter of makin' it more tolerable...the back enclosure contains the heat from the propane disc (about the same output as a Mr Heater)...w/out the back, there's no containment and you'll have to be about 6" from the heater to feel it...across the front, the center and both sides were made to roll up and strap up for open-air running when needed.
1 1/2 hrs down I-85 to the ramp...I never even considered leaving the top up for the 60-70 mph tow...I did stow all the snap on curtains inside the truck to keep them warm for easier attachment...
BTW..that top is still intact today and is available for sale...cut specifically for a '74, I wouldn't recommend for 80s or later models as there were some changes...COULD be altered, but again, I wouldn't recommend it...
Can't believe these the only pics I have...l don't have it, but Rick said he was looking for a new home for it last we talked...