I'll answer your first question - a 350P is a completely different engine from a 400, so you cannot just bore out the 350P to a 4.12" bore size (you'll hit water jackets). They do share many parts, like the 3.75" stroke crank, rods, oil pump, windage tray, timing cover, pullies and accessory brackets, but that's it.
I scored a running 71 350P cheap, but I still replaced the rear main seal, cam, lifters and timing chain and repainted it- no point in dropping a dirty, leaky engine into a car. This engine was going to go into my 72 Bird, but the 406 in there now is still happy, so why pull it? I'm planning to use it as a temporary engine to power my 76 TA.
If you have alot of deposits on the heads, you could pull the intake, valley pan, put rags or carboard and ductape in the lifter valley and scrape it clean, but IMO loosening up all of this crud is a bad idea- the small particles could clog a lifter or oil passage. Unfortunately the only way to properly clean heads is to remove and disassemble them, and then thermal bake or soak them in a hot tank. I wouldn't spend big money on 350P heads, unless I was going to put them on a 455 or upgrade to larger valves. The no.94 heads on my 350P have small valves and 96cc chambers and would need alot of work to have them make power, so they can just stay on the shortblock as is and continue to run happily on 87 octane.
Geno
