I would suggest carving the guitar cake, and then placing it on top of a full sheet cake (two 1/2 sheets put together). The full sheet should feed about 80 people, and the guitar will serve the rest of the people, or more, depending on how big you make it.
You can put the guitar on it's own board, so that you can remove it first and set it aside. Then you can cut up the sheet cake, and save that awesome guitar for last!
If you have enough time, check ebay for a guitar shaped pan. I have one, it's one of my mom's from back in the day. I'm not sure if wilton still sells a guitar pan or not, but it would be easier than cutting it out of a sheet cake.
Wilton still sells the guitar pan but it is pretty small. You might only get about 20 servings from it depending on whether you cut and fill or only make it one layer with frosting.
I'm not sure if this will answer your question, but at least it will give you an idea to work with.
I did a guitar cake (see pics). This cake was 33" long and 12" wide. I carved it from one 12 X 18. It served approx. 30 (and I understand the pieces were a bit generous). My guess would be that it could feed closer to 40 for a wedding.
So, a little more than two 12 X 18's...????
Good luck...this thing is going to be massive!
Maybe you could just cut and do the body of the guitar and leave off the long neck. I have seen several people do cakes like that on this site.
Quote by @%username% on %date%
%body%