Speedo cable connector?

Stock 51 Merc speedo and cable. 350 Turbo trans with speedo cable. Cables inside the casings are the same size. Any idea how I can connect to the cables together? Some kind of juction box? Will the speedometer readings be completely out to lunch? I was thinking something like the old cruise control units GM used in the late 70s early 80s. Cable in, cable out.
Looking for any ideas.
Thanks!

 

I checked the Speedo Head Tread and the Trans Tread & whent to NAPA & got one & it Works Great
os course you need a Tread Guage to Measure it
if its off you can go to a ****** Shop & get a Different Gear
Hope this Helps
I have a sbc with a Powerglide in my Merc

 

Most cable ends are pretty universal, you only run into trouble where some c**** use a clip on the speedo end instead of screwing on -

You only need to change the gear if the rearend ratio is different from the car the transmission was originally in -

 

try Carquest # CA 3002 for 63" , # CA 3004 for 83"

to get your speedo drive/driven gears right go here:

http://tciauto.com/Products/TechInfo/speedo_gears.asp

 

So what are you saying? Don't join the cables? Just get a longer one? I must admit. I never thought of that. Not sure how the Merc speedo attaches, I'll have to crawl under the dash and look. Napa shows a speedo cable 84" long (should do it) for a 81' Caprice. $18.00. So long as it attaches to the back of my Merc speedometer I'm good to go.....

 

IF the stock original cable will reach the Chevy transmission ... it will work. Then you just have to get the speedometer drive gears RIGHT in the Turbo 350.



The above photo is of a 700R4 but a Turbo 350 works the same way.

 

Yes, that I know. Done many a ****** swap. And many plastic gears swapped to get the speedo to read correct. I whant my speedo to work in my current ride. 1972 turbo 350 + 1951 Merc = ?

 

The Caprice cable won't work as it uses a clip on the speedo end. the Merc cable has a nut that screws on the back of the speedo. Almost all real parts houses sell "universal" cables with housings in various lengths. That is what both 36Threewindow and Rusty NewYorker both said.

As they said, If the car still has the original cable housing take it out and figure how much longer a cable you need and take it and the the housing that fits the turbo 350 to the parts house and lay them on the counter and tell them what you need. Napa or Carquest not Autodummys.

I did just that when I put the 350 Buick and turbo 350 in My 51 Merc.

 

Ok, got it. Heading out to the shop now to pull the cables off.
Thanks for all the input guys!!