Good Morning to you all

Looking for some tricks to getting the slotted head bolts removed to get the motor plate off (pics below for reference). I have tried some heat, tapping the end of a big screwdriver while turning, and an impact driver (not a gun, but the tool you hit with a hammer) which only twisted the bit. I am going to get a bigger hardened bit for the driver and try that, but hoping one of you may have a trick that works.

Thanks in advance
Jeremy
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1955 3600 father and son project