First off I hope these mechanics have actual experience with those older 855s, the simple fact they are saying they are worried about what they'll find concerns me, big deal you got cavitated liner by the sounds of it, cracked possibly, throw a cylinder kit(slug and jug) into just that hole at min, the one beside as well since that head is off already and given its age, roll a set of shells in the bottom end rods and mains, reset the top end and go. Whether the other two heads need to be pulled and the other four holes done is something that can only be determined by actually looking at the condition of the engine and I can't do that long distance. It takes an experienced set of hands to properly set one of these up so I am hoping your mechanics are familiar with these engines. Not as sensitive to settings as a two stroke Jimmy but it does effect performance.
Anything that needs to be done can all be done in the truck.
One thing that concerns me is if it has been run much with water in the oil, that could somewhat raise **** with bearing journals on crank, but again can't say with actually looking.
76 is probably Small Cam, reason for names given is a SC has a 1/2" smaller cam shaft diameter than a Big Cam. Same basic block externally and engines do use some common components but not all. Heads are the same I believe, but followers and rocker boxes ect are diff. Yes they can be interchanged as a complete assembly fairly easy.
Guess this is kind of short and blunt, sorry bout that, I'm tired lol.
Anything else, ask, tomorrow is another day.
x2 on that plan, given that low usage I would just make it work again without fluids going where they shouldn't. And it dont really matter a rats ar** if the rest of the engine isnt 100%, 95% is good enough. As long as it starts and runs for 1000m(1600km) you dont need or want to be spending a lot on it.