I'm nearly positive that in 68 Mopar only used a single pin switch so I think the auto parts store was wrong. It wasn't until 69 they used the 3 prong switch. I find auto parts stores are wrong a lot on old iron like ours.
As far as the switch leaking, I have worked on numerous trans. leaks over the last 28 yrs. and never found even one switch leaking through the plastic. Not saying it can't happen but I don't think it's a very high possibility.