Well the 66 Q code should be a 428. But from the tag and the crank stroke measurement it sure sounds like a 390. It could have been a service replacement even under warranty at the time. It would not be the first time a Ford dealer had installed another displacent engine to substitute a blown or defective part unknown to the customer. OR someone went to a lot of trouble to find a 66 390 to replace it. Either way the car should of had a 428 when it rolled off the line. If it was sold to you as numbers matching it ain't. The ribs you saw on that supposed CJ block were used on every service and production block from 72 and later. So you were looking at a 428 service block not an actual production CJ block. But in some concours circles the service blocks are accepeted as original replacements. The blocks date code is key sometimes. But your Q code bird should have a std 428 not the CJ. If you have to know exactly what you have you'll need to remove one head to measure the blocks cyl bore and either remove the oilpan to ID the crank or get an exact measurement. You can put #1 on TDC, remove #1 and 4 sparkplugs and get your measurement there. You use a lenght of wood dowel or stiff wire and insert into each plug hole and mark. Be sure to use a common edge as a guide...the meausre the distance between the 2 marks. This will be your stroke. The Q code 428's used std parts as far as heads and intake. So those would possibly be the same on either a 390 or Q code 428. But the 66 428 had 345 hp vs the Z code bird 315 hp 390. The C6ME block casting number was generic and used on many different bore blocks from the day. This makes IDing the block difficult. As far as making a 428 out of a 390 it depends on what cyl wall cores were used when the block was cast. If it does have thicker wall cores and coreshift providing you could. But it's a crapshoot till you measure for sure. Most 390's have std 390 wall cores but a few have been found with thicker 428 walls. JMO....G.