wireing newbie query,..

,,have to patch in a new Painless Wireing T2 pre-wired 7 circuit fuse block and need to make some assumptions on what I can tear out/remove and where to start patching into the old circuit-Just starting out this weekend and I have an Haynes manual on the XJ and I think that I need to locate a place to start ,maybe patch in the power behind the key/start switch on my ol 84 XJ with the keyed switch and additional start button-Any advice/suggestions-TIA

We need more info. What are you planning to run off of this fuse block? Why would you want to splice into existing circuits? Give me a better idea of what you want to accomplish and I can give you some good ideas.