I would consider my new build custom. It's a savage 10 action, but it is being worked over by the smith, customizing the action. Stock is a McMillan, customized with a loggerhead cheek. Most likely custom painted by me. Bedded by a good friend, further customizing the mating of action and stock.
New recoil lug, precision ground and trued by the smith, aftermarket trigger, barrel etc...on and on
About the only thing you can touch on the gun that is factory is the bottom metal and safety.
I can call it a savage if I want, truth is, it is far more custom than savage, if something goes wrong, it won't be savage that is liable anymore.
It isn't "purist" custom, as in a stock carved by me, action machined by me etc...
I think things become custom when they can't be described otherwise, I really couldn't sell this new gun as a savage, it isn't, won't even have the nut.
But I don't care, I just want it to shoot well, if it doesn't do that then it doesn't matter what you call it, it's just an expensive stick.