such a thing?
...aftermarket...
...leave in stock master position position
have a 56 power parts but not planing to goto Power

 

I won't say it can't be done (anything can be done with enough money and effort) but it would not be easy leaving the master cylinder in the stock position. If you put it on the firewall it would be pretty easy but would require lots of plumbing.

 

I've done it on my "56. I had to weld a mounting bracket and modify some parts to get it reliable.
I also switched to front disc brakes and added a proportionating valve.
One tough thing is bleeding the system because the master cylinder is below the rest of the system.
If you want to tackle it I can provide pictures and part numbers of what I used.
Wally