I havent done any soda blasting so its a tough call on cleaning those areas and seams. Ive used paint remover on the painted areas that are hard to get at then it only takes light pressure from the wire wheel (on the end of an electric drill) to remove the paint that can be stubborn and hard to get at.
Those seams your talking about on the trunk above/near the drop down.....those are tough. I replaced that panel there (trunk extensions I believe they are called) with new. Since you have that roll bar welded to the bumper/tail panel brace that make things tough.....cause I would recommend just replacing that panel and the bumper support brace.....would be an easy job with how far apart your car is.....but that hefty roll bar welded to the bumper to trunk to tail panel brace makes things complicated.
http://www.parts123.com/parts123/yb....Z5Z5Z50000136d