If all preambles are the same size ( 2 bytes ) and always start wit 0x2a, you shouldn't have a problem. I've ran the packet through ethereal and here's the packetdump :

Frame 1 (57 on wire, 57 captured)

    Arrival Time: Jan 12, 2003 00:41:23.8484

    Time delta from previous packet: 0.000000 seconds

    Frame Number: 1

    Packet Length: 57 bytes

    Capture Length: 57 bytes

Ethernet II

    Destination: ff:ff:ff:ff:ff:ff (ff:ff:ff:ff:ff:ff)

    Source: 00:00:00:00:00:00 (00:00:00:00:00:00)

    Type: ARP (0x0806)

    Trailer: 806401100001000000000000204545

Address Resolution Protocol (request)

    Hardware type: Ethernet (0x0001)

    Protocol type: IP (0x0800)

    Hardware size: 6

    Protocol size: 4

    Opcode: request (0x0001)

    Sender hardware address: 00:00:00:00:00:00

    Sender protocol address: 192.168.240.100

    Target hardware address: 00:00:00:00:00:00

    Target protocol address: 192.168.240.105



   0  ffff ffff ffff 0000 0000 0000 0806 0001   ................

  10  0800 0604 0001 0000 0000 0000 c0a8 f064   ...............d

  20  0000 0000 0000 c0a8 f069 8064 0110 0001   .........i.d....

  30  0000 0000 0000 2045 45                    ...... EE