Gene,
I am probably the seller of these heads. It is a good idea to ask around, especially for parts like these. All I have to add to this is that these heads have the cast in spring perch cups, screw in studs and the slotted push rod holes. The indeed have the C5OE casting and the date code of 5L5 (December 5th, 1965 BTW), which would make them appropriate for a '66 car. Other stampings on the heads include the number "19" on one of the accessory bosses as well as markings of "4V", "65" and "289". They do not have the "HP" markings, but it is my understanding that not all hipo heads had this marking. At the same time, the hipo heads were the only heads with cast in spring perches and screw in studs.
These heads are complete with valves and springs. They need valve work and they have been repaired once before I bought them. I have no idea how this happened, but one of the intake bolt bosses that is not cast in the intake deck broke. The owner before me had it welded and retapped. It is a very professional job and can not be seen once the intake is on. I ran it like this for over 10 years w/o any problems at all.
While I am new to this forum, I have contributed heavily for quite a few years to the Fordnatics mailing list and I have sold and bought several things to/from members on that list. I recommend that you extend your search to this list as well for more checkup if it will make you and your friend feel better. I know that I would be doing the same thing if I was in your shoes, so this does not offend me one bit. In fact when it comes time to purchase some of the high dollar items for my new motor, I am sure I will be doing the same thing.
Hope to talk to you and your friend soon.
Michael Bradburn
[email protected]
'91 Mustang LX 5.0 Conv.
'67 Mustang 377
"She got 5 speed, 4.11's, 10.13 in a quarter mile,
trick gas, full blown nitrous, speed queen
with a smile" - Sammy Hagar