There have been 3 major iterations of the Ruger M77, the original M77, aka "tang safety," the Mark II, and now the Hawkeye. The Tang Safety replacement by the Mark II was the largest change, new stock profile, change from push feed to CRF, new trigger, safety position change... it was almost a completely new rifle... The transition from the Mark II to the Hawkeye was very slight, a new stock profile and a new trigger... if you're building it as a custom, the differences between the two won't matter, functionally, and you'll only need to check the right box on your stock and trigger when ordering (the stock inlet really is the same, just a slight difference in the trigger area, which is basically imperceptible).
There are a lot of smiths who won't take on a Ruger for any work, even stock bedding. I made good money for several years by referrals from other smiths, since I was the only guy in town happy to bed them. The receivers are hard as woodpecker lips, so many guys don't care to burn up tooling to true them either. Of course, many, many smiths don't have fixtures for Ruger receivers, and don't care to invest in one for a single project, because the market of would-be Ruger customers is tiny. Aftermarket support for the Rugers really isn't bad, lots of stock options, triggers, and all major barrel makers will turn a short chambered barrel for you, many of them will even do the installation.
Ruger's typically don't make a great platform for custom rifles compared to other brands. The integral scope mounts are great for guys who like them, not great for everyone else. The aftermarket triggers out there are great hunting triggers, but they don't really compare for a CE or HVR. I've not heard of anyone sleeving a Ruger bolt, but I know all of them I've owned sure could use it. Many guys also don't care for the way Ruger bolts feel when open, guys like dogs to wag their tails, not their bolt guns. I've rebuilt a handful of Rugers, or 20 some... so I'm not saying DON'T do it, but if it's just a passing fancy because you own the action already, I'd say your best value is to sell the Ruger as is and use the proceeds towards something else. If you REALLY like the rifle, then welcome to the club, prepare yourself for criticism, and for a really nice rifle once it's all said and done.