Straight swap as the 1500 is a 12v motor (12v flywheel into 12v gearbox, as they are so 'labelled' coming from a 12v volt car as opposed to the 67 and earlier 6 volt models)... Just make sure the 1600 has a generator rather than an alternator, as the voltage regulator setup would be ready to go, unless u wanted to upgrade, then its a little more work (nuffin to get frightened of). Mind you, the 'alternator' issue shouldn't be a prob unless the engine has been changed, as they had not released alternators as a stock thing on 1600's in 1970.
You can just use ur original 1500 gearbox. its bellhousing and input shaft will happily take your 1600. HOWEVER! If its a 1500 gearbox, it will be 'swing axle' rear suspension. If the gearbox is from a 1970 super bug, which, being a 1600 i'm pretty sure it will be otherwise its a 1300 late bug, (have i got this right guys?) then the gearbox will be rear trailing arm suspension and as such, ur chassis will need an irs conversion. (messy). Not really necessary, unless ur pro racing, or wanna do some serious offroading.
If u want your suspension to handle good, restore/replace any sus components. Worn bushes, ball joints (being 1500 pan), shocks and steering damper will all make it an interesting ride from what i have heard on avd. If u aren't familiar in all of them, take it to a vw mechanic and get them to overlook it all, particularly it being a safety concern.
full interior? we talking headlining, carpet and seat covers? OR we talking upholster?
All those components can be bought from ebay, or ur vw parts supplier.
Parts suppliers include:
www.classicveedub.com.au (NSW based)
www.mickmotors.com.au
Theres a place called vintage vee dub which i believe is south too... (google is ur friend)
www.cip1.com (which is from the u.s.a, and are not bad in price but postage can sometimes be a killer, not all the time. Really you get the savings if u are buying a good lump sum from them )