The MS290 and other saws Stihl makes that have plastic based lower crankcase halves, are a PITA to rebuild. The flat rate for a cylinder replacement is like 2 hrs. for an experienced tech WITH clean components, so being unfamiliar will obviously add to that time, as well as cleaning. Flat rates are often averages, so you decide...
Anyway, you'll have to clean all of the sealant from the mounting flanges, and you MUST, ABSOLUTELY MUST use a high-end sealant such as what Stihl sells, Dirko. OR, some other ANAEROBIC, FUEL-IMPERVIOUS sealant ELSE the seal may fail and the saw will probably toast itself running lean from a vacuum leak. REGULAR RTV sealant that costs like $4 a tube IS INSUFFICIENT and inferior for the task at hand.
Any engine that splits at the crank centerline is going to be a bit of a challenge to assemble. You have to compress the rings and get them into the cylinder, while trying not to get the pre-applied sealant over everything.
We usually condemn any of this type of saw needing a cylinder, as it's just too much work and expensive. The MS290, while supplies last (is being phased out) is $399. It's replacement, the MS291 is $100 more.
I don't understand what you mean by blade mounting bolt...could you mean bar-stud?
To be honest, even though I've worked on Stihl products for 30 years, I probably have only rebuilt 1 center-line split crankcase saw, and maybe a handful of other units like trimmers or blowers that also split center-line. I am very familiar with the 290 in all other regards though...will try to help but am quite busy myself so if I don't answer right away have some patience.