Kodiak
As always, my first suggestion is to look for a Kodiak Mk.III or Mk.IV. If your truck didn't come with one, I wouldn't wast time with the round smiths. I have no experience with the Mansfield heater, and I've heard they perform well, but they are big bucks.
The kodiak Mk.IV has the fan mounted in the engine bay, with the fresh air intake routed to the front grill. This arrangement provides a forced fresh air feed. I use this as the 'low-speed' and it keeps the interior of my 109 really toasty. Flick on the fan and the heat is overwhelming. No intake hole in the wing either, so maybe you could do similar with the Mansfield...
Regards,
Tom P.
1965 exMoD 109
1995 RRC LWB w/EAS