Every FJ40 my family has owned ('74, '78, and '82) has had the factory solder give way on either the upper or lower radiator tank, or both.
Take it to a real radiator shop, have the tanks removed, the tubes rodded, and the tanks resoldered. Comb the fines, new paint job, and you're ready for years of trouble-free service. Have a half-assed job done, and you'll be fixing it every six months until you have it done right.
Every one we've done right has lasted for as long as we've owned the cruiser.
I put a new OEM in my '82 last year, and almost talked myself into having it resoldered right out of the Toyota box. Nah, getting home from a split radiator seam is half the fun!