The dipstick was actually out of the block a couple inches and oil had been escaping until it turned the oil light on (…) The crankcase venting is 100% stock 1964 vintage.
If you're building up enough crankcase pressure to push oil out the dipstick tube, something's wrong…bigly. That would be severely excessive crankcase pressure, meaning you've got a ton of blowby and/or your crankcase ventilation system is not functioning correctly.
Is there a positive way to retain the stick in the tube? Right now it just drops into place without any resistance. I checked the dipstick on a 340 I have here and it's the same way. I never remembet seeing any grommet or o-ring on any of my Mopars.
Right, because there has never been a need for positive dipstick retention. The drop-into-place dipstick remains in place just fine…unless there's much too much crankcase pressure. Clamping the dipstick in place in a situation like yours would be like stuffing a cork in the tailpipe to "cure" exhaust smoke.
I welded a couple small plates at the front and rear of the sump (…) maybe that's causing windage. I can remove the pan and cut out the baffles if needed.