I would advise against blasting the crankcase halves with anything harder than walnut shells. Soda blasting will profile the surface...the as-cast surface gets obliterated and the now-textured surface will grab & hold dirt like never before. At that point, they may as well be metalfinished & polished.
Polished aluminum is a polarizing topic; you either love it or hate it, virtually no middle ground. In the case of engine castings, it's also a paradox...the most work to maintain and the easiest finish to maintain, over the long haul. Polished surfaces have to be massaged periodically, to keep clean & oxidation free...IOW maintained. OTOH, the maintenance means 2-3 brief sessions with a cotton rag and aluminum polish (or NevRDull wadding polish) annually, which has the added benefit of actually improving the polish level over time. Polished aluminum is solvent & chip proof, it doesn't yellow and is unaffected by engine heat. When it gets scratched, the imperfection(s) can be polished back to the pre-injury condition, and usually while left in-situ.
Pieces such as the clutch cover, flywheel cover, points inspection cover and cam cover take a lot of work to metalfinish & polish but they're not bad tokeep look like new subsequently. These are also the pieces most likely to be scuffed from normal riding. They can easily be painted at any time, should you change your mind. The crankcase halves are a nightmare to polish & maintain and once done, there's no reversing the process. For that reason I'd leave the case halves as-cast, unless building a full-on custom.
