Dave, those Ronstan RF58 swivel jammers look the same as ours, the camcleat base is pretty solid – you will not see that flex, also remember that the blocks are taking a lot of the strain.
The ratchet blocks we use are RWO – switchable on/off – http://www.sailboats.co.uk/Product~Rwo_Mini_Ratchet_45mm_Single_Port_R5437.html
But I would add that we have had 2 fail over 8 years. I would guess you will want some supporting springs or collars. Cheek blocks might be okay – just depends on the run of the sheets – with a fixed block it will be more difficult to get a clear run for the sheet.
I cannot comment on modern auto-ratchet blocks, personally I like to know if they are on or off.
You will want a backing pad if you are fixing to a GRP thwart