Hi there
I do like someone who is willing to go that bit extra to make a machine perform as they want and not just accept that the OEM got it right. Looking at your images I would say that these guides are the same as a Sabre 350 and you have no worries about enlarging the slot to get the lower guides fitted as thats what Record have done on the 350. It is worth noting that Record now sell the Sabre fence and rail as an upgrade for the BS350 & 400 so will they get round to selling the guides as an upgrade as well? I suspect that they may have concerns because of modifications around safety guards and implications involving PUWER and the machinery directive although I bet there are many out there in domestic workshops who have taken the hacksaw to something!
I have also seen or read somewhere that the lower guides are far less important than the upper guides, anyone any thoughts or expand on this? I think that both sets of guides act together to support the blade either side of where it is cutting but there is so much differing info out there.