I wrote a tool years ago for this, it may be helpful.
There are a few different calculators on the page, but I tried to describe what they do ..

I *think the 3rd one down does what you want.
I like math, but not in the middle of recording .. so I created this for quick dirty work.

http://tools.masteringmatters.com/

/since your song is 6/8 I'd half the existing tempo and use 3/4 in the top two boxes .. then put 4/4 in the bottom box ; *should show the new 4/4 tempo once calculated

steps -
enter half the existing tempo in top box (if it is currently 132 tempo enter 66 .. 132 divided by 2 = 66)
enter a 3 in second box
enter a 4 in third box
Click Calculate
Solution shows under the 'new tempo' label

I should write it so it does all the odd time sigs, but like I said it was a quickie project to save myself time.

I do not work here, but the benefits are still awesome
Make your sound your own!