Notice to all BUYERS. You are buying at your own risk. If you do not know the seller then please be cautious and look for suspicious activity. If they ask you to mail a check/money order or pay with PayPal using the Friends and Family option, that is a RED FLAG. Your purchase will not be protected. Only pay with credit card or PayPal using the goods and services options. Do not fall for the excuse that they don't want to pay the PayPal fees. They need to account for that in their sales price.
To all USERS that think they need to police the site. New users that come to sell items are not always selling stolen merchandise. No vigilante or lynch mob mentality is permitted. You will be given infraction points if you accuse someone of selling stolen merchandise without hard evidence. You must have the stolen item report with a serial number and/or pictures that match the for sale item to make such accusations. Alert the moderators with your evidence and they will take care of any transgressions. If you do not you will be given enough points for a 3 day time out.
066/660 both take the same "medium" Stihl mount (3003). The "large" mount (3002) is for saws like 076, 084, 088/880, but can be made to work on a 660 with an adapter and possibly a new oil hole drilled into the bar. In either case, changing the sprocket tip from 404 to 3/8 is cake, especially on the oregon bars as there is only 1 rivet. 5 minute job for a drill and a ball peen hammer.
I personally think you should keep the bar and change the tip. Good luck with whatever you decide.
Look for the model# of your bar - should be stamped on there someplace. If it's starts with "RN", it's probably part# 30853. If it's "PM" or "RW", it's 39520. Many of the site sponsors should have these in stock.
[EDIT] Lots of saw dealers also carry Oregon products. You can probably find one locally, and maybe even get them to install it for you if they do repairs.