GT...how will I know..I know nothing of Spectra's...It does have like "lock out" button
or something next to the lever....Anymore help will be great... 
Got pics?

Some of the control boxes dont like to be run with only one or the other cable, the Quicksilver boxes work OK, as do the Morse ones, but some of the older Teleflex units are major pains in the buttocks to get to work without leaving the cable in it...
I have a throttle pedal and a Morse lever if needed, but the shift lever only needs like an 1 1/2" hole for installation, you probably have like a 2" hole.
GT