I agree that is where the challenges are. The main issue, aside from the risk of making a mess of getting the crankpin to enter each counterweight straight, is the need to end up with the two main bearings in line with each other. The crankpin is just a parallel, it does not have a key or other device to align it, and even if it did, it wouldn't align it well enough. A combination of a good jig, and a lot of experience, is what it takes to get a decent outcome without having to correct a major error to get it straight. Don't even think about it without a proper hydraulic press - and I don't mean a glorified car jack, either - but you still need a jig, and some V blocks and dial indicators for checking and correcting. The correcting part calls for another proper jig.
See why I don't bother with built-up crankshafts? For a home repairer, they are not really a rebuildable component.
