What product do you recommend I use for removing undercoating from the bottom of my 69 SS before I sandblast it? Thanks.

Lots of opininons on this... I used a heat gun and a scraper. I've heard others use oven cleaner and a scraper.

This stuff Peal Away works. (http://www.vandykes.com/product/sa110011/peel-away-7-quart)
It's a paste that you brush on, then cover it with the supplied fabric, which is a lot like Tyvec. Let itm sit over night and peal it off in the morning. It softens it enough to peal off the majority of the undercoating, then you can scrape the rest with a scraper. I've seen this stuff work.

I used heavy duty easy-off oven cleaner and a putty knife. Followed up with a red scotch brite pad and some simple green. I think it worked out about as well as I would've hoped.

I just went thru this on my 70.. Paint reducer worked the best but fumes killed us! Ended up using easy-off oven cleaner and a putty knife.

Another vote for heat and scrape.

Heat and scrape

Again, heat and scrape. When it cools it hardens and sweeps up easily. No mess.

I used an electric heat gun and a few different sized putty knives. For followup I used rags and laquer thinner (bought a gallon). The laquer thinner worked much better when I also used the heat gun to warm up the residue left from the scraping.