Reading Haf-E's reply has me realizing that I (mis)read your note in regards to putting a Westy roof on a standard VW van ( misread as the 77 was already a Westy and you want to mount it a on a US van.
So you have a standard tin-top Bay window (aka Loaf) van and want to install a Westy Top - correct. For a Vanagon, I could give you advice and point you to some websites, for the 77, I am sure it can be done, but I am not sure of the best way.
For the Vanagon, the roof under the Westy top is NOT the same, so the best way is the hard way, removing welds and the entire top skin and then fitting the Westy donor roof metal and then mounting the fiberglass top and luggage carrier. But plenty simply buy there top, cut a hole in the roof and skip the top bead, or fabricate something a bit homemade. The place to ask is on TheSamba.com
Personally, unless you have a family, I'd
skip the pop-top for now. Get the van going (1st thing is to change the rule lines, as these vans burn every year), then add the camper interior (personally, I like the Weekender set-up, no stove or fridge, perhaps a sink/icebox) and see how much you like the van before committing to the pop-top. Then if you are still on board, research and do the pop-top correctly.
Fuel line info - http://www.benplace.com/fuel_line.htm - remember 1st item and use SAE30R9 hose only!!! 7mm is the desired size, but you will have to use 5/16" - it's will still work.