Joined: Jan 2010
Posts: 9,835
Likes: 1,816
From: Northern California
Bikes: Cheltenham-Pedersen racer, Boulder F/S Paris-Roubaix, Varsity racer, '52 Christophe, '62 Continental, '92 Merckx, '75 Limongi, '76 Presto, '72 Gitane SC, '71 Schwinn SS, etc.
A $15 pair of 6sp-indexed Shimano thumb shifters can be clamped onto a road handlebar.
An alloy bar is 2mm bigger diameter than the mtb bars, so a longer clamp bolt is needed.
Most alloy bars are even bigger a couple of inches to either side of the clamp area, so an even longer, bent bolt can be used together with a nut (so the bent bolt won't need to be rotated in order to tighten it).
I've done this mod to a few road bikes and it works well.
Note that one lever will need to be positioned slightly higher than the other, so the cable exits don't interfere.