It would probably be very helpful to have one site that has info on how to extract the latest ml64, 64 bit libraries, user manuals and sample 64 bit code. The only one I've seen is http://www.codegurus.be/codegurus/Programming/assembler&win64_en.htm
Goasm's syntax is similar to Masm. Here is about writing 64-bit programs :
http://www.jorgon.freeserve.co.uk/GoasmHelp/64bits.htm