I put a Harley five gallon tank on my 650. I too wanted to keep the stock speedo and put it on the bars. I did and it works just fine. I went to a local "pre-owned" bike parts yard. The guy who knows the yard like he knows his wife took me to an older three cyl. Suzuki (early to mid 80's I think) and told me that the tach cable should work. I don't remember the bike model, but how many threes are there? The thing you have to look for is the end that goes into the block. Make sure the threaded part and the insert part are the same as your shorter stock cable. I didn't measure it, but it's more than long enough to route up under the tank to anywhere you want to go.
Pull out the inner cable, clean it well, put some motor oil on it for lubrication and slide it back into it's sheath. I also put details on here in a previous posting about what I came up with for mounting the speedo on the bars. A pretty tidy and clean look if I do say so myself. Cheap too. Here's the link to see pix of what I did with mine.
http://bikepics.com/members/jazzcarter/00savage/Jack