That's the big question !
How do you tunnels without the need of pinging a remote host in the target network ? Our costumers get hanged on a regular base because of this issue. 10 points to the first answer !
You do not need to authenticate ( its optional )Just use ntp server command.
also in "ntp server 10.0.6.5 key 1 source outside"
use source as inside.
i think you will also have to enable "management-access inside" to make it work .
Check the following example for PIX to PIX Ipsec-NTP
You can make a Router in your network as ntp master and sync clocks on PIX and other non-cisco devices to the router.
You can setup keepalives on VPN end .
isakmp keepalive 30 2
crypto isakmp keepalive 10 periodic
If it does not resolve it and you need some kind of traffic then you can configure NTP across the VPN link (source it from private interface so that it is interesting traffic for VPN).