Hi Alex,
So I'm pretty new to this whole thing myself, but from my experience in dressing myself to achieve a feminine form in my selfies, I'd have to say that it's all about the waist and hips.
But first, a comment about your build. I'm pre-HRT, 5' 11", around 40" overbust, 38" underbust, 33" natural waist, 39" hips and I consider myself on the slim side. So the first thing I'd say is...don't get hung up on being an ex-rugby player!...you're actually very slim for your height and it seems to me that you're going to be best served by trying to improve your hip:waist ratio through whatever means you can so that visually it takes some focus away from your top half and balances you out a bit more.
The reason I say that is...even though you specifically mentioned your top half, your dimensions for your bust are tiny for your height. What I mean by that is, if you were to go and look for and find a 6' 4" cis-girl, she would most definitely have a bust of that size or bigger. That's not the bit that you need to alter to achieve believable proportions (and this is a blessing because I get the impression many MtFs struggle with having big chests!).
So.
In your situation, I think I'd avoid tight jeans completely, because they tend to make it obvious what we don't have (hips!).
If you must go for jeans, then consider getting some kind of hip pads to add a couple of inches. I generally try to achieve my feminine look with a minimum of padding (generally just breast forms), but I also picked up some cheap "boy shorts with pads" on amazon to help on those days that I need help making an outfit as feminine as I would like. There's no shame in that and many cis-girls do the same to make the most of what they have.
I recently posted my new (mainly empty!) FB profile for comments with pics, so I'll repost it here to give you an idea of what I go for with my particular dimensions, and hopefully it might provide you with some inspiration...(there's one pic there with me in leather leggings, so that's probably the most relevant photo):
https://www.facebook.com/media/set/?set=a.108866379513878.1073741829.100011715484935&type=1&l=ad122da0f4I hope that helps in some way!
Orielle
x
P.S. I know I didn't really touch on tops. For me, the biggest and best thing I do with my top, is keep my shoulders as far back as they go as that makes me look narrower, helps me elongate my neck and generally look more feminine. Most cis-girls have way better posture than cis-boys IMO.