diver,,,,,,,,no, Do not remove the top plates, They tie things together and leaving them will make it all the stronger,,,,,,,JMHO on the matter........
May this new year be kind, and fair to all..........
getting a better picture here originally i thought the beam was going in such that it would hold up the center where the second story was and the load associated with it now i see that the beam (as it is) is to hold up the "top" of the upper floor - or - the roof structure...... if i am correct i think (remember i am no engineer) that a double 2 x 12 would work just fine - put a 4 x 4 under each end - and if not positioned over lower support - ensure support from under the posts