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The motor you have is NOT too heavy. 247 lbs.
https://www.whalercentral.com/article...icle_id=77

The 15' hulls are rated for max of 70hp.

Many members have the Yamaha F70 which is only 5 lbs heavier than your motor at 253 lbs.

Apparently your transom is using different holes that do not conform to the BIA standard.
You may have to redrill the lower transom holes so any motor will fit.
https://www.whalercentral.com/article...icle_id=82

Are you positive this is a 15' hull?

Here are some 13' transom variations. Maybe this is what you have?
A 13' model is rated for 40hp with remote steering.
https://www.whalercentral.com/viewpag...age_id=155