How to get rid of understeer in a 1970 340

Given everything else you've listed so far, I might start to suspect the tires. Tall sidewalls can give the car a "vague" feeling in response to your inputs, can come across as steering or alignment issues. Your 225/60/14's are pretty short, so there really isn't any more sidewall than on a 15" tire, but it could still be adding to the issue. Not to mention the generally lousy tire compounds that are available in those sizes.

Based on everything you've done already, after you confirm the alignment I'd go back to recommending 1"+ torsion bars, front and rear sway bars, and a better set of shocks. The Bilstein RCD's make for awesome shocks, and they're not as expensive as some of the adjustables out there. After that, you're pretty much just left with tires.