1: different rear end gearing - also, work on the brakes (disc conversions are prevalent.
2: you can add an oil filter
3: yes, it involves new panels and a good bit of work. A 5-window cab may be easier
4: Floor shifter? (or see 5)
5: split exhaust manifolds, multi-carbed manifolds, 4-barrel manifolds, turbos, etc. are all available. Best driving is an EFI conversion, do a search in the HI-PO section.
Welcome to the bolt.