Steel into steel? Bench vise with an old saddle. Get it to rotate and it will come out. Don't grab onto the stays. Head tube and seat tube. Penetrating oil of your choice first. If it doesn't start turning quickly, stop, then soak it some more. I have bent frames with the brute force method, so don't overdo it and don't grab the stays.
Brute force method not recommended for aluminum into steel as you have other issues.
I'm assuming you have a straight round seat post, no integrated clamp. You could clamp onto the seat post into the vise, but you will tend to distort the seat post.
You can still find six speed bikes brand new at the box store of your choice. Those bikes have kept the continued production of 6 and 7 speed freewheels for those of us with older bikes. So myself, I am thankful they are still out there.
No workshop? Find a co op in your area if possible. Or a neighbor with a bench vise. Or a friend at work?
Aluminum can corrode into steel, forming a somewhat permanent bond. Those can be a lot more work to free up. You are fortunate with steel into steel.
Last edited by wrk101; 06-13-22 at 09:21 PM.