32/64-bit transaction IDs?

I need a guaranteed ordering of transaction IDs as they are queued for
asyncronous replication.  I'd use the 32-bit internal transaction ID from
GetTransactionId(), but 32-bits will not suffice for the logic I'd like to
use.  A 64-bit ID would work well enough.

Further, I am uncertain those 32-bit transaction IDs represent a correctly
ordered sequence of transactions.

Do the internal transaction IDs provide the correct transaction order?

Are 64-bit transaction IDs available?

If not, what other tricks are there to get guaranteed ordering?

Ed