Plan to do above to make room for brake power boost, Has anybody done that? Concerned mainly as to how to best vent that setup in trunk.  thanks for all advice

I did this to a 1965 Mustang Shelby Clone once.  I used welding cable inside of a rubber hose I hid along the inside door jams.  I mounted a battery tray to the trunk floor just like Shelby did.   I used a no maintenance battery with a false top to make it look stock. No vent required.

Putting the battery inside the trunk, you're not restricted by size/type of battery-no need to vent/w sealed battery. However, use heavy-gauge cables, and use grommets/thick electrical tape, etc. if cables have to pass through/near any sharp metal. Make absolutely certain the battery is secure, and use a box to protect other items in your trunk as well.  

Phil, unless you never put anything in the trunk of that Mustang, I would have found a better, more secure way to route the yellow wire.

Why not just use one of the high powered mini batteries and leave it under the hood where it belongs?

John - "The Hoosier Hurricane"

Nice setup Phil. I presently have Interstate vented battery thru caps Had hoped to use vent tubes as done on BMW's. Looks like I'll have to go with sealed battery. I have a vented battery box which I plan to mount in the spare tire well. Thanks again...Richard