I was in the same boat, sheetmetal dash. Mine stopped at the speaker grill though. I guess you saw the two adjustable brackets on either side of the bottom edge of the glass. I made a point of scribing a mark so the brackets would go back exactly and the glass would fit again when I was done. The problem I had doing it myself ($15 worth for tape and primer was a good deal) was the layout of the buytl tape. Just like Whitey, you can see the tape above the trim on the lower edge just barely. Not ugly but not pure either. The top alignment was difficult and made me think the seal would leak. There is a very thin area to work with at the top edge of the opening and the tape has to be just right. I used the 5/16 butyl tape and had a little problem with the trim fitting back in place...so I waited for a sunny day (2-3 days later) and parked the car in the sun, then stood on the hood in my socks and pushed as evenly as possible to "seat" the glass against the tape/seal/whatever. Paint Thinner cleans the butyl mess up real good off of the old glass and you definitely need a friend to help place the glass on for the final time. All in all, I would do it again, but I am a cheap skate and I know what to watch for now.
p.s. make sure you cover your paint REALLY good with something that won't burn through when you weld your new dash in...I missed a little spot and have a few little microscopic holes melted through my clear coat.