Boundary alignment (follow up)

Hi everyone,

Two days ago I asked something about boundary alignment, and I have been trying many stuff in GRC to solve this. I am stuck on this for days and any help will be appreciated. Below is a description of what I wish to do.

1) My transmission and receiving chain are like this: signal source -> constellation modulator -> Adalm Pluto Tx-> Antenna -> Adalm Pluto Rx -> Polyphase Clock Sync -> CMA equalizer -> Costas Loop -> decoding. As for modulation scheme, I am using QPSK.
2) Now I just want to transmit one byte output from the signal source, and following with one byte of a known pattern, and then one byte output from the signal source, etc. Then at the receiver, because i already know the known pattern, I need to sync to the known pattern in order to align the bytes, which will allow me to hear clean audio at the receiver.
3) There are three things I am looking for:
a. I have been looking for a SIMPLE way to prepend a known pattern at the transmitter, and send it with the output bytes of a signal source. Is there any block that might work to do this in GRC?
b. I have been trying to see if i can write an embedded python block to prepend a known pattern, but I am having a hard time trying to understand the input_items of the block, and how i can add a known pattern iteratively.
c. If i can successfully send the known pattern with my data to the receiver, is there any easy way to parse at the receiver?

Thank you in advance!
Lannan