Thanks for the info everyone.
The only plugin I guess is absolutely necessary to have working in 64-bit at the moment is rampbug fix plugin.
It may be possible to just swap out DHooks for DynoHooks which does support 64-bit according to the Sourcemod Github issue thread. I wonder if it's an easy replacement in this regard?
Also, a specific issue for dedicated servers, I seem to get around 20 real ping (not sure why, the server is running sv_lan 1 and on the same machine as my client), compared to 0 ping on listen servers. It makes jump timing, especially syncs, quite different. Is this to be expected as well?