Viterbi decoder on a bit stream where the preamble is also coded I hope this is not off-topic for this site.

In my problem, Viterbi decoder is used for decoding convolutional-coded (rate = 1/2, constraint length = 7) messages.

A message is 250 bits where the first 8 bits are 1/3 of a preamble. There are several types of messages. A message is received as 500 bits because of the coding, which means, the preamble is also coded.

Since there is no place marker other than the coded preamble, I am in trouble with finding the first bit of the 500 bits to start decoding.

I would really appreciate any suggestions and comments.
Is the 500-bit encoded message part of a continuous stream being decoded by the Viterbi decoder, or is it a 500-bit packet that is decoded individually?   If part of a stream, then decoding continuously and searching for the preamble indicates the beginning of the message.   If packetized, then the decoder memory needs to be managed so that the first decoded bit location is known.   This will be dependent on the decoder implementation.