Welcome Steve.
First, don't beat yourself up about the price for the trans rebuild.
My 1986 4WD B2's trans. (the same one that came out went back in) cost that much plus tax to do--2 1/2 years ago.
And they only did the trans -no radiator or lines were replaced.
(It did come with a 2 year unlimited mileage full coverage warranty from them including any needed labor.)
About that SEAM leak.
Having experience the same problem myself, here's how I fixed it, its so simple that its hard to believe they haven't done it, and then pressure tested the cooling system-to check for any problems.
First, know that there is a thick, resilient gasket between the side tanks and the core. The aluminum tabs are there to compress the gasket.
So (in my mind) the obvious thing to try is to tighten them to compress the gasket more in the area where where its leaking.
So, what I did was just that and the leak stopped-and the entire cooling system was pressure tested-by me. Actually,it took 2 tries, because I was being conservative. Thinking ahead, I slightly tightened ALL the tabs.
Once the radiator was out of the vehicle for proper access, I took a good pair of Vice Grips with flat faced jaws (actual brand) and with them gradually increased the tension until I could get a repeatable setting for the "crimp thickness" of the existing maximum amount of compression the tabs had. Some of the tabs took more "hand squeeze pressure" than others to close the wrench, so before even doing any adjusting, the gasket was most likely physically thicker/less compressed at those.
OK, so then I marked the adjusting k-n-o-b on the wrench to index it for a starting point, and VERY SLIGHTLY increased the pressure adjustment. (It was by something like 1/4 of a turn at a time.) I worked around the radiator on both tanks, closing the wrench on each one-and being sure to check that the setting didn't change.
The first time I went around the tanks, there was a very noticeable variation in hand grip pressure needed to close the wrench. So I took that as meaning some were still looser than others. Tightened the wrench SETTING by another increment, and went around the tanks again. That second time the hand squeeze pressure needed to close the wrench was within a much narrower range.
Being a conservative type, I reinstalled the radiator, filled the cooling system and did a system pressure check at 2 psi ABOVE the radiator cap's specification. The leak was MUCH SLOWER, but was still there-coolant just barely oozing out. So I took the radiator out and reset the wrench to the next incremental adjustment and once more did all the tabs, reinstalled the radiator and redid the pressure testing, saw nothing leaking. Came out in 10 minutes and the pressure was holding steady. Put a big piece of corrugated cardboard under the engine compartment, checked to see that the front carpet was dry, and came back in an hour-still no change. Came back a few hours later-still no change. Checked the next morning-pressure holding, dry cardboard and carpet.
That was about 8 years ago.
I've changed heater cores, heater hoses, radiator hoses, even the thermostat, gasket, (and radiator cap too, I believe) since then. Still no radiator leak.
That reminds me.
Have the radiator cap pressure tested too. (I had done mine-it was ok)
If a system is, for example, supposed to have 12psi cap and there is a 15psi TESTING cap on it, (even a malfunctioning 12psi cap) whenever the engine gets warm the system is operating at a higher pressure than it is designed to hold.
OVERPRESSSURE can easily cause leaks, and if a radiator tank's gasket compression just happens to be the weakest link in the pressurized system, that's where it would leak.
Overpressure could even do that to a NEW radiator .