It is no problem to convert the Flushing line to BMT standards. It is BUILT to BMT standards, and already had gap fillers that can be removed (or at least used to?) that can be removed to accomodate B division cars.
What cannot accomodate B Div equipment is the Steinway Tunnel system (the "S" curves west of the bridge, the tunnel and the tracks into Times Square.
My plan is to Bring the (R) train out to Main Street on the Flushing Line. Of course there is NO WAY (here or elsewhere) that this service can even begin to supply the capacity that these neighborhoods require until AFTER the Northern Boulevard Subway is built. (See my Myrtle-Fifth Avenue Subway plans for details.)
Once this is built, the needs of service on the Flushing Line will be altered so that it will be well withing the capacity of the (R) train running on Broadway via the 60th Street Tunnel.
The Times Square - Steinway lines will then then be connected to a new service running on the LIE ROW and making stops at several large park and ride facilities to be built at stretegic points. Some of the trains will continue via the Montauk Line and thence south on the Rockaway line to the Rockaways, replacing all (A) service to the Rockaways. [All (A) service will be extended along Liberty Avenue, thence bending northward on Supthin Boulevard to a terminal perpendicular to Hillside Avenue.
Now go raise me the money, and I'll build the subways for you!
: ) Elias