Just got around to doing this today, replacing a universal chrome hotrod style mirror that the PO of my recently purchased 73' had installed.
I used a few different techniques than Socorob, so I figured I'd offer my advice to anyone thinking of doing this in the future.
First thing, I used a dremel to cut most of the way through the housing and then used a soldering iron to melt/cut the last bit. This saved me the stress of possibly gouging the delicate silvering, and yet I was able to press right up to the glass. Separating the halves was very easy this way, though I'd only recommend doing this somewhere well ventilated.
Second change was how I installed the mirror into the housing. Socorob is absolutely correct when he said this is hard to do. What worked for me was pressing the mirror in by hand, just to get at least one of the long sides into the groove. I then placed the assembly mirror side down on a soft towel on my garage floor. At this point, I took another towel, folded it repeatedly until I had a nice inch thick pillow.
...here's where things got a little nerve-racking! Using the 'pillow' I had made, I gave the housing 3 or 4 firm, but cushioned taps with a mallet. Presto! Mirror was snapped into place in under a minute, no water necessary.
Thanks again Socorob for doing all the leg work figuring this out. I walked into the JY today with zero tools and walked out ten minutes later with two mirrors.
Total cost was $14 .