Bicycle Mechanics - Stuck stem. Hacksaw blade won't fit!

I decided to cut the very stuck seatpost and stem out of an old Gitane I recently bought for $10. The seatpost is going ok, not there yet but I feel like I will get it eventually.

Once I cut the top of the stem off I realize my hacksaw blade won't fit to cut a slice down the length of it. What are my options now? I've lost all my leverage by cutting the top off, and I was risking bending the fork anyway. I have not tried ammonia yet (Al stem stuck in steel steerer tube.)

I've been know to take a sheet metal shears and cut the blade down to the width I need

I don't know if you want to save the fork, but at least you should be able to remove the fork from the frame now that the top of the stem is removed by unscrewing the top of the headset. Assuming your old Giante has a threaded fork, you should probably be concentrating on trying to move the wedge at the base of the stem. Try running a rod larger with a slightly larger diameter than the wedge bolt (ie one that will fit) and pound on it for a while. Once you have loosened the wedge you should have an easier time removing the remnant of the stem.

You can pull the fork, and since there's no cosmetic issue with the steerer, warm it with a torch.

If the bottom is open, and you can either flip the wedge over, you can put the headset locknut back on, support it straddling the jaws of a bench vise, and use a long punch to drive the stem out. (the wedge must be flipped over for this to work otherwise you'll be expanding the stem making it worse) This method only works with conical wedge stems, not with Japanese style angle wedges.

Do the ammonia, pound on the wedge or expansion bolt, get a huge visegrips. The ammonia really does wonders to free up steel/aluminum interfaces. Pour it up from the bottom and the top, give it time to work.

Update: I've made two cuts the length of the stem (180 degrees from eachother) and I can still barely budge it with vise grips and lot of hammering. I think I will try the ammonia next as there is at least a gap now to pour it into.

FB - I like the wedge idea, I hadn't thought of that. I managed to get the wedge out the bottom so my next step will be hammering the step out from the bottom.

You could also try drilling it out. Start with a drill bit slightly larger than the existing opening and increase the drill bit size by a 1/16th each time until it's thin enough to cut with a blade in 3 or 4 places. Do not try and drill out the entire stem as the hole tends to drift off center.