I'm in the process of rebuilding a 1990 251.
As you did I also bought sight unseen. Exterior wise looked decent.
Upholstery needed to be done. I got video of it running. It's a 454 Mag, rebuilt with upgrades.
Had a 251 before and really like the boat, and the price was right.
Well, it's been two years. Stringers were rotten and wet all the way forward.
Transom surprisingly is solid and dry.
So at this point, stringers and floors are all new. New fuel tank as the old was pitted real bad. Rebuilt engine again as I found 2 dead cylinders. Valves where a mess. Looked to have been overheated.
But getting there. Upholstery is next. Maybe see water this summer.
I will have way more into it than it will ever be worth. But it's what I want.
It is definitely takes time. We are doing everything we can to get this thing going by the end of summer but also not trying to skip anything important. That sounds like a great project, have you made a post with pictures? If you have please attach a link.