To put road cranks in your oversized BB shell you will need the conversion, but you can pick up a set of 1-piece cranks and an unsealed BB for them for very very cheap, I'd guess around 30 bucks all told. Just go to your LBS and tell 'em what you're looking for, they'll understand.

A tandem cruiser changes things up a bit. Tandems typically have one off-center (eccentric) bottom bracket for tensioning the chain between the captain and stoker. I haven't messed with too many tandems so I'd probably have to see pictures of what you're working with to tell you what you'll need. Expatriate rides a tandem, he might have some input that doesn't consist of "go to a BMX site and buy expensive BMX stuff for your non-BMX bike that you're trying to revive simply and cheaply" as well.