Even with a 60-70 foot line, there's still no reason not to use the casts you may already know. Maybe it's not as traditional or something to cast a double spey with a 65' D-loop, but you can still do it. I mentioned this in a similar thread a week or so ago, but one thing that is helping me on my double-spey and snap-t casts (which is all I really know how to do, even using 60-70 foot lines) is to allow my arms to rise up and away from my body as I finish building the D-loop, right before I transition to the forward stroke. I kinda have to wait a little longer to let the loop form and turn around me, when compared to casting shorter fatter heads.
Also, when I bought my first line for the Clearwater, it was a 600-something grain Skagit head and a total beast compared to whatever dinosaur line my father-in-law was punishing me with at the time. And I still remember Poppy yelling to me as I walked out of his shop: "Stay off the gas!" because those things don't need much added speed to collect a bunch of energy. With the longer head, and coming from a shorter set-up, you may find you'll need to speed your tempo up a bit to keep the line moving.
But mostly, get out on the river and play around. See if you can figure it out by being creative and open-minded, trying a bunch of stuff that seems like it might work. When I first started kayaking, I had no clue what I was doing, or what I was "supposed" to do. But the joy of being on the water eventually led me to understand I had the freedom to try whatever seemed interesting, even if it ended in failure. There are things to learn from other people, but there are also things to learn from yourself. This "playing around, trying things", in my mind actually leads to as much learning as formal instruction, as long as it's done with some kind of focus. Early years are invaluable, because we can learn without the stress of having struggled over the same **** for what seems like ever. Later on, all that focused play will translate into intuition.