I know a number of cases where SiteA --302--> SiteB and the SiteA remained on the same position in SERPs (but the meta description and <title> shown is from SiteB). A visitor lands on SiteB of course.
Hence hopefully you may be able to test conversion on SiteB by implementing 302 redirect without impacting SERPs.
However, one thing that you will not be able to test is whether the CTR from SERPs is higher/lower/the same when URL of SiteB is shown in SERPS (which would be if you use 301 redirect, but may not be with 302 redirect when the old site still remains in SERPs).
Sgt_Kickaxe's suggestion of taking a handful pages to test is a good one.