OK I have never heard of any radiator shop, "recommending" any kind of stop leak, unless they are even more crooked than most others, see they all clog up every tiny little hole in the system, it is a temporary fix at best. At worst it will clog up things so bad that even a flush and fill may not fix unless done repeatedly. Something tells me the original owner used that stuff already and that may be the very reason you are getting no heat. Think about how a stop leak of any kind works, it plugs up holes and it doesn't have a clue what ones need to be clogged up so it plugs them all.
Whenever we fill a Fiero cooling system, you must burp your baby and I am not being funny here, think about you kids when they were on the bottle and if you didn't get the gas out by burping them and how cranky they got. Well in our cars it is a lot similar only they get cranky in the form of over heating and that can be even harder on a car than a kid.
read the ogres cave and listen, he has some great advise there.
if you want to do a back yard mechanics flush and fill of your heater core take both heater core hoses off the heater core and cut the end off a garden hose and clamp it onto one of the heater core tubes and another section onto the other tube and into a clean white 5 gallon bucket. You must use hose clamps, you must or you are going to get a hose blow off the cores tube. Now place one hose in to 5 gallon bucket and have a helper hold that in there or it may blow right out of it when flushing. I use a small hose shut off on the end of a good hose so I am not destroying a good 50' hose and then after the lever shut off, available at any hardware store or China Mart for 50 cents, then a hose clamp onto a small section of hear hose to the core with another clamp on the core tube.
now charge your garden hose and then attach the shut off to the core, open the valve slowly, it shouldn't take a few seconds for the water to circulate threw the core and out to the water bucket. When it does start flowing have your helper watch what comes out of the other tube, I bet its all junk, if anything comes out at all. that is most likely the stop leak, or could even be peace's of the old hoses that have deteriorated over the decades.
I didn't see if you said there was a leak under the dash, if there was that is your core leaking. they usually leak inside the passenger compartment. now if no leaks or no flow into the bucket open the valve a little more. eventually you will get some flow into the bucket. make sure you don't open the valve all the way until you get some flow into the bucket or you risk doing even more damage to the heater core.
once you get some good flow run it for a while until about a half a bucket of water then open it up all the way and let it run for a second or so and shut it off. now check the floor inside the if its wet you have a leaking heater core, if not its fine. check the 5 gallon bucket for crap.
Technology is great when it works,
and one big pain in the ass when it doesn't
Detroit iron rules all the rest are just toys.