IMO Either have the harmonic balancer/pulley welded to the crank where it sits, or expect to put a bunch of work and $$ into your I6. Once you get into the engine, you are going to find more and more "precautionary" fixes. Not to mention that, in order to get the crankshaft out (to swap it with another, or to repair your balancer issue), the tranny/t-case will have to be pulled.
If you don't just "cut your losses" and have the balancer welded, you WILL have to pull the engine. Even if you, were able to pull the crank, with the engine in. There is no way you will get it back in correctly. But If you try, I would pay to watch,... it won't be pretty.
The I6's are fine engines, and will run literally forever if you don't "tamper" with them. But the economics will work out much more in your favor going to a 350, or most any Chev (V8 or V6). Once the initial parts are accumulated and installed, you could literally swap in and out a dozen junkyard V8's for the price of rebuilding an F or 2F once. If you are going for a rebuild, buy one from MAF or SOR, or someone who's done a bunch. Landcruiser parts cost way to much (in comparison to Chevy,... or F*rd parts, for that matter), to have a mechanic botch a rebuild.