First off, I love your truck!!

Your idea of mounting a second tank beside your current overflow tank will be the easiest. If that does not work out, perhaps one of the following ideas will be helpful.

Is there space to mount a recovery tank on the firewall? I'm thinking of something similar to the 1970's truck tank at the top of the firewall just under the hood. The mounts may need to be designed so the tank is easily removed to provide access to the distributor.

Does the entire recovery tank need to be above the radiator? I thought only the top of the recovery tank or the fill point needed to be that high. If so, could you mount the overflow tank under the fender where you have space, but can not access it easily. Then run a pipe or hose up to the firewall or some other location so it will be up high enough? This is assuming you can mount the recovery tank in a location where it can be seen under the fender, even it you can not reach it, so you can tell how much coolant is in the tank. Perhaps you could look in over top of the front wheel to see the tank? A tank is this location would need to be made out of something strong so stuff kicked up by the front wheel will not rupture the tank.

Taking care of all the little issues after a non-factory engine swap can be time consuming. My father performed many such swaps. Sometimes the smallest problems can really test a guy's persistence!