I need to find a youth model stock that would fit that action.
With respect, I would contact Fajen, Boyd, etc. and explain to them that I wanted to buy a youth sized stock to fit the M-77.
I would also suggest that you look for a used Ruger M-77 stock, perhaps one that has already been shortened.
A company named "Ram Line," has produced a number of synthetic type stocks for numerous models of rifles. You might see if they have a stock that you can shorten.
I would personally focus on finding a used M-77 stock. Opening the barrel channel up to accept the heavy .22-250 barrel would not be to big a deal. Just use a dowen and various grades of sandpaper until you have relieved the wood to the right depth, then give it a coat of clear resin or some sort of water proof polyurethane varnish. That will help keep the stock stable in changing weather conditions.
I hope your project is completely successful! Sincerely. Brucev.