Re: Big Block 'fit' questions??
Doing the same thing to my 69, so I can help some.
There are big block specific frame mounts (the two bolt on "towers" in the cradle under the engine) for the fist gens. Offsets the engine to the passenger side about an inch to resolve clearance issues on the driver side. Pulls the engine forward a little too, I think (it's early). If you need more clearance at the firewall, you can always slide the entire subframe forward. You should probably check that the subframe is aligned within Fisher Body specs anyway. Do a search on here for subframe alignment to find a thread that has the Fisher Body diagram for 68. If you can't find it, I can scan it and post it or email it to you.
Water pump depends on what accessory brackets and pulleys you want to use. If you are buying new 68 BB brackets and pulleys, you'll want a short pump because that's what 68 had and is what you'll need to get things lined up. If you are using the 402 brackets and pulleys, it'll be a long pump. I don't think there is any clearance isssues with either pump in a 68.
The rad/fan issue will have to be answered by someone else. My 402 is so mild, I'm going to try a stock replacement rad and thermo clutch fan to start.
On the power steering parts, what do you need? Are you using an existing box, and just need a pump? Lots of us have installed a 3rd gen steering box to get rid of the "pinky" light steering of the 1st gen box, but I'm not sure what you're asking for. I'm just running 69 BB brackets and a 69 replacement pump.
Unless you plan to use some really tall valve covers, a stock size (11") booster fits fine when the BB stands and mounts are used. Mine has plenty of clearance.
You'll need a BB trans crossmember, and that may be a little bit of a pain depending on the trans you run. The BB crossmembers had the same offset (1" or so) as the engine mounts to keep the engine/trans straight in the car. BB manual cars used one cross (looks just like the small block cross except the cutout for the mount bolt holes is shaped different and the holes are off set) that you can use for a manual or TH350. BB auto cars (TH400) used a tubular cross.
Wait, you're worried about room for a fill tube, so you're running an auto. Which one?
Have you dealt with the heater core yet? BB uses a different core than the 6/small block cars; heater hoses on a BB come out of the center of the box cover.
Almost forgot the coil springs. Big block springs are taller to compensate for the extra weight. You may have clearance and handling issues, but some folks just run the 6/SB springs.
69 'vert project big block/TKO 600 RR
68 'vert driver RS clone -- gone!