According to the following I *think* you should be OK with single link, assuming I correctly remember that the limit is 165 MHz for single link.
Can you pastebin the entire log ?(II) RADEONHD(0): Supported additional Video Mode:
(II) RADEONHD(0): clock: 156.8 MHz Image Size: 510 x 287 mm
(II) RADEONHD(0): h_active: 2048 h_sync: 2096 h_sync_end 2128 h_blank_end 2208 h_border: 0
(II) RADEONHD(0): v_active: 1152 v_sync: 1155 v_sync_end 1160 v_blanking: 1185 v_border: 0
(II) RADEONHD(0): Ranges: V min: 56 V max: 60 Hz, H min: 30 H max: 81 kHz, PixClock max 160 MHz
I'm not sure that 1.2.4 completely supports the 4670 though, will check... EDIT - yeah, should be OK. I also checked the H and V frequencies and confirmed that they were in range as well. Might be something related to "TMDS Coherent Mode", I think radeonhd defaults to incoherent; if so then trying coherent would be a good next step (TMDS coherent mode refers to the detailed timing relationship between clock edges and data transitions, nothing to do with the developers state of mind ).
The "radeon" driver you used (6.8.0) is much too old to support 4670; we're at 6.12.2 today.