I put a Painless harness in my '68 and don't remember a problem with the fit of the fuse block. I may have had to open up the mounting holes slightly, but the fuse block slipped in perfectly from both sides.
I was (and still am) a novice with electrical and I found the install to be frustrating due to my lack of knowledge of wiring, but also my lack of knowledge of the Firebird in general. I have never driven my bird or had any electrical power to it. So I didn't know how anything was supposed to work.
But I did get through it on my own and have learned a ton, mostly because the kit was detailed and complete. Each wire is labeled to where it is going. You will have to take a lot of connections from your original harness and splice them into the painless harness. This is the only wiring I have ever done, so I don’t have a baseline, but I would suggest their 18 circuit kit.
