Unless you want to redesign it, or have one fabbed in aluminum, you're stuck with it. BUT : You may not be getting all it has to give. Before I flipped mine, I had noticed that it didn't blow very hot. While I was rebuilding it, I discovered that much of the evaporator core was blocked by leaves and the heater core had caught some, too. (Totally tearing your truck apart can be very informative!) I realized that the leaves were coming through the slots in the wiper valance, which is why later trucks have small holes. When choosing junkyard body parts, I made sure to get the "holey" valance. I cleaned both cores with a water hose (pressure washers will bend the vanes) and emptied all the leaves from the ducts. There is a rubber drain valve (full of leaves) at each front corner of the firewall, a galvanized plate with 4 screws just below the dash behind each kick panel (passenger side full of leaves), the evaporator core can be checked thru the resistor plate hole and dislodged (without opening the system) enough to hose it out, and the heater core is easy to remove after removing the glove box (not the door). I also noticed that my blower motor was sluggish, so I replaced it. (Along that line, check the connectors on the back of the A/C control to see if they're overheating and melting.) After all this, my heater will scald anyone in the passenger seat, and is plenty for me.
The point is that your truck is old, and how often have you done ANY maintenance on the heater?? It's not too small, it just needs some cleaning.
The first thing to check since you're in a hurry is the evaporator core. You can see a little and feel a lot thru the resistor hole between the blower motor and the accumulator. If it's clogged, pull the cover from the other side of the air box in the engine compartment (about 5 screws there and 1 nut at the bottom of the firewall inside the passenger footwell), and hose backward thru it until you can see thru every hole.
To see what I did to mine, go to http://www.webphotos.com/listPicture...w&album_id=217
. If it challenges you for a password, ignore that, look down to the left and put "steve" in the "E-mail/Nickname" field and select "My 83 Bronco".
83 Custom w/95 4.9L EFI, 78 NP435, 83 NP208, 83 D44IFS 3.07, 87 Ford 8.8ABS 3.08, Michelin LTX AT 31x10.50R15