Maytag Washer With Belt Falling Off

I have a Maytag washing machine (belt driven). The belt keeps falling off, so looked underneath and it appears the motor can be moved back and forth…I assume, so one can change the belt. So I put the belt back on and moved the motor back to make the belt relatively tight, BUT what I believe are the two nuts to tighten the motor down in place, I can turn them all day and the do not tighten. The motor keeps moving back and forth in its slots.

This is probably poorly worded, but not sure how else to state it. Any help is greatly appreciated. Is there a special tool to tighten down these nuts, or is the assembly just worn out or is the crazy thing desingned for the motor to ride along these rails? Thanks.

What is the model number?

HAV2557AWW Thanks.