I wouldn't use heat, myself.

Have you thought about drilling a bunch of small holes, thru the rubber, first, then try and "punch it out"?

The few other brands of pickups, that I've done; I removed leaf spring pack and put assembly into a hydraulic press. Honestly, most times, I've been installing a lift kit, so  leaf spring packs were new = didn't have to screw around with old bushings.

 On my older Toyota rear springs; I fought with them, a little, when I put in a bushing kit = I had to remove one complete spring pack to get bushing changed.(once out on floor, I got a good swing with sledge hammer and it came out.)