If you are going to duplicate your own stock make it into a perfect pattern stock and pay the little extra to have a very close duplication made of it. A 90 or 95% stock sounds like an easy to finish blank. I'd far rather pay double for a 97-98% inlet stock. None are truly 100% drop in inlet unless they are sloppy as an pig in boots. A excellent job will involve more scraping than cutting.
The 90% you might as well start out with a square blank. It saves you almost nothing. A 95% is better but still leaves a lot of areas to get exactly right. And I hate to see any inlet job which ends up with a glass bedding to take up the slop from a bad inlet job.
