Anyways it was a trick question. The answer is 'nothing'.
You've chosen two periods of clothing history where the most desired shape was elongated and androgynous. Ideally suited to someone tall without womanly boobs or hips. I'll look at solutions for both periods, although if you're carrying a tummy, the twenties styles may be more flattering.
On your top half you want a high neckline - and a close-fitting black polo neck worn untucked would be a stylish look, although possibly more 'beat' than 'mod'. Alternatives might be a long-sleeved, fitted shirt in a tiny floral print. Liberty (above) are the ultimate for these, but new they cost £75. Look out for second hand ones on ebay, or lookalikes in other stores. To ensure the look is female, you need close fitting lines, but that doesn't mean going a size smaller. The right tops will be tailored in at the side seams, but will fit comfortably. If you're stuck, a local dressmaker should be able to take a man's shirt in somewhat.
The second look incorporates a skirt, but it needn't be a mini, as Leslie Ash demonstrates above centre. The over all profile is clean lines and slightly flared from the shoulder to the hem, as this will help to skim over your tummy. Avoid anything with a horizontal seam at the bust, you need boobs to carry that off. The best thing to do is to search on ebay. I found the dress above right, which has a 40inch chest, currently going for £9.99 (auction ends in two days). Note, high neck and long sleeves - those are the areas you want to keep covered. Underneath your dress, you'll need opaque tights and ballet pumps, again.
With twenties style, you have to decide whether you want a replica outfit, or something influence by those looks. Replica outfits like the one above can be made from patterns available in the US and copied from original designs. Note the opaque tights again, and you'll also want T-bar or Mary Jane shoes with a low heel. Dance suppliers stock these inexpensively up to a size 10 (although dance shoes tend to come up a size smaller). If that isn't big enough, I've found some almost right ones - the Glinda range here comes in a host of colours and is available up to a size 13.
If you want to add a twenties accent to daywear, think about the sillhouette. Drop waistlines, with or without fullness above, are de riguer, and as with the mod look, the over all line is straight, although there may be some flaring towards the hem. This might be as simple as wearing a long plain jumper/tunic over some wide leg trousers, but you can add interest with some long knotted beads, or a long silk scarf, a la Isadora Duncan. If you go for plain coloured clothes you can really go to town on these accessories - try a chunky bangle too (a hinged one will go over larger hands) this one is perfect.
This season however, there are a lot of twenties influenced outfits on the high street. Blouson-style tops and dropped waist tunics will give the right silhouette above the waist. You could team the top above (£10 in the sale at Long Tall Sally - also available in grey) or even something as modern as this low waist, batwing jumper (also at Long Tall Sally - £25) with a long, bias cut (nearly straight) skirt. Again Long tall Sally comes up trumps with the devore number above in grey (£25) or brown (£15).
Note to all my girls - there are some good bargains to be had at LTS - especially for those who find their tops for work are never long enough!
edit - apologies for the odd formatting of this entry. HTML editing is beyond me, but I promise normal service will be reumed next time.