Waughoo... Did we help answer your questions.
Have you been able to check which type of GT40 heads you have? Not sure what state of disassembly your engine is in (you mentioned valve covers off) but the casting number for the heads can be found on the underside of the head in the lifter valley. Pictured engine has the 1.7 pedestal mount roller rockers like a Cobra would have from factory.
The way to identify which type of GT40 heads you have are:
- 3 or 4 bars on the front (93-95 Cobra, Early Explorer, Lightning will be 3 bar). GT40P is 4 bar
- Top outside corner of head, does it have 1 or 2 holes? Pic here is of Explorer head with 1 hole. My post above shows 2 hole with air injection port. 2 hole is 93 Cobra (not sure on 94-95 or Lightning)
- Casting number of heads. Found on the underside of the heads in the lifter valley approximately where shown. 93-95 Cobra, Lightning and some Explorer are F3ZE. Published identification charts list F1ZE for Explorer 3 bar heads. I personally have not seen any (my sample size is 3 sets of Explorer 3-bar heads, all F3ZE's)
- Exhaust port air injection bump and holes. All F3ZE have the bump, 93 Cobra has the holes, Explorer heads do not have the holes.
Hope all the info shared in this thread has helped to answer your original question. What have you determined you have?