Hey Robert I just found the same ribs on the engine in my 63 Galaxie. I am hoping it is a 390 but it was a rebuilt engine a few years ago and I have the engine tag it was bored .040 by national engines so who knows what it is.

I will have to measure the stroke and since the engine is together I cannot measure the bore.

Glad to hear yours is a 390 I hope mine is also.