Hi Adhitha,
I haven't used the "OFDM Insert Preamble" module before. But in my understanding, in tx_ofdm.grc, "OFDM Carrier Allocator" does the job of "OFDM insert preamble". It uses "sync word" instead of "preamble". I think they are the same thing. As for FFT length, keep it the same as your other OFDM modules. Note that the length of a sync word must be the same as FFT length.
Regards,
Edwin