bigolbug:
I am considering a 1200 baud rate. I will have a total of 6336 bits to transmit and so considering transmitting at ruffly 6 bits per baud.
You haven't mentioned much that I can see about your proposed modulation method. Are you thinking of an PSK or FSK type arrangement? 6 bits per baud is probably pretty ambitious. If it were me, I would start with some sort of simple binary keying -- On-Off Keying or a 2-frequency shift -- and then use a known robust encoding, like the one used by VirtualWire. This would yield something like 0.3 to 0.5 bits/baud, but with some built-in error checking.