Ken,
Congratulations on your find! I'd say you definitely did the right thing. That is a 1967 model. GE did some odd things with serial numbers in the 1960's -- making it hard to date some 1960's GE appliances even using GE's own dating table. But they used letters in the model number that indicated the year, and "C" is for 1967.
The small oven on my 1969 40" GE P7 range, which I bought earlier this year, was even dirtier than your oven. I used "professional strength" EZ-Off cleaner, over repeated sessions, until it was finally clean. Just be sure to protect the floor under the oven door, per the instructions, and I recommend you wear a respirator -- or at least a dust mask with a breathing vent -- while you're spraying and cleaning the oven.
Good luck finding the rotisserie and grill. You'd think those accessories would have been kept in the range's storage drawer -- but it seems people find some reason to store them elsewhere, and they end up getting separated from the stove. That happened on my 1969 range, too, and I had to use eBay -- plus the kindness of one AW.org member -- to reassemble a set of accessories. All I'm lacking now is the rotisserie handle, but I found a modern replacement that fits.
Dean[this post was last edited: 10/26/2015-20:25]