Dylan, you haven't really told us much about your heat exchanger. It's capacity, rating, etc. They are not all the same. I tried to "design" a system using theory and --- hope you're sitting down --- math and physics. Theoretically, I could calculate everything up front and build the heat exchange myself using copper tubing.
Unfortunately, I was missing one important variable - the flow rate of the engine coolant through the heat exchange(HE). In other words, how many gpm of coolant going into the HE. The other variables, gpm of the fresh water (my 12VDC pump rating), the temp of the coolant, temp of the fresh water and even the coefficient for the copper and area of contact were all known. But I had no good way to get a flow rate for the coolant into the HE.
By the way, the area of contact is the critical part of the HE design. How much surface area separating the Hot and cold sides. The picture yhou provided doesn't really show me what's happening inside the HE.
Sadly, I'm lazy and have never got around to back calculating the coolant flow rate using the empirical data I've collected using my own shower. It shouldn't be hard, but I lost interest since the shower works great.
Sooooooo....
Dylan, pull the HE out of the truck and hook up your garden hose to one port. Run the hose at a constant flow rate into a bucket. Measure and time it and you'll have a gallons per minute (gpm). You might even try to set the hose at about the same flow as the 12VDC pump you picked up. Then switch the hose to another port and get a gpm from that port using the same "setting" on the hose. Are these flows the same or is one side more restrictive? I think you should plumb your coolant through the "least" restrictive side.
Installing into the FJ60....
I tapped into my coolant system -before- the rear heater. If you look at the firewall, there are two metal tubes coming up from the rear heater. I think the one on the right (looking from the front or the one towards the center of the rig if that makes more sense) is the supply and the one on the left is the return. So... tie into the coolant hose before the supply, between it and the engine. This goes to the "coolant in" on the HE, the "coolant out" will go to the supply tube of the rear heater continuing the circuit. Again, this should be the least restrictive side of the HE from the previous test. Savvvy?
Next supply fresh water to the end of the HE near the "coolant out" so the "hot fresh" exits the HE near the "coolant in". That's the crossflow idea.
Obviously, you want to take extra care with the coolant connections since this is now part of your permanent cooling system and a leak will leave you on the side of the road. Set the heater slide to HOT on the rear heater and warm up the engine. As noted above, the HE should become as hot as the top of the radiator ~200 degrees. You can do this before plumbing the fresh water if you want to test it.
Now, plumb up the fresh water side. Using your garden hose is fine to test. Run water through the HE and use the bucket and stopwatch to calculate the gpm. Get a thermometer and measure the temp of the water going in and after to get a "delta" temp. A slower flow will get a higher "delta temp". Higher flows will get a low delta temp.
Get some numbers and let us know what you find.
You can't do much about the flow rate of the coolant - except idle the engine faster for more or close the heater slide switch for less. So you're left with controlling the flow rate from your 12VDC fresh water pump. If you don't get enough flow and "delta temp" you'll need a larger HE.
Whew, sorry for the long winded post...
FWIW, the HE in Knuckles is rated at a "1/2 Ton" and I get about 50 degrees "delta" temp with a 3gpm. I had a "1/3 Ton" HE that got about 30 degrees - not enough for me. I spare you why they are rated in tons. Hint: Mass and BTUs.
I have a R&M HE in Ruby and use a 1.5 gpm pump. It works well too, good heat but less flowrate. The larger HE and pump on Knuckles gives it the "Binford" edge.
