Lag bolts in pure lead are IMHO a bad idea. In pig iron it's OK, but in pure lead I'd be worried about the bolts drawing the thread. There are numbers out there on how much "bury" of lag bolt is needed in lead to prevent this, so it is done, and I remember being surprised at how little it was. Still I wouldn't sleep well like that. It should have been obvious when tapping which is the case here. On larger boats cheek bolts are often used for this, but that is a process which has it's own drawbacks. I would suggest appropriately sized SS or bronze rod run all the way through and threaded as needed. Use a forstner bit to bore the counterbore for the nuts and washers on the bottom, then fair over with epoxy
. Boring the holes is a nightmare, if you drop the keel lining up properly becomes a problem, if you don't you need much more than jobber length bits, and have to resort to extensions. Sometimes you run into a scenario where the extension and bit length needed is more than the available headroom! But only on big boats usually. Jobber length bits and extensions are expensive. So is SS or Bronze rod. But it is a fairly simple job for DIY, just a bear to pull off right.