I have had some success with a hand-held impact driver, penetrant oil for a few days prior will help. If you have access to the back side, use a propane or MAPP torch to heat the screw.

1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy