I interchange 18F252, 18F242, 18F2420, 18F2520 and 18F2620 with 16F876's all the time, on the same boards with a DS1307 installed (although I use different pins to you). From memory, other than CONFIG FUSES and the Weak Pull-Up's having a different Register, there's not too many other changes to look out for. Can you try another 18F PIC? what versions of PBP and MPLAB/MPASM are you using?