The cap kit I used was for standard 4900 series so thats all the same. I looked into swapping out the neck board from a standard, and although it looked identical the part number on it was different as were the transistors and tube socket. These parts looked like they could be upgraded as the boards themselves looked identical.
It's funny you should chime in, because I was actually more interested in the conversion of a 25k5501 to a 25k5515. Have you come across a manual for the 25k5501 yet?
Nope. But I was comparing my 5501 with a 4901 the other day (same IC's, caps, flybacks, etc) and the only noticeable difference I could find was the extra little board and stuff mounted on the left side wall.