I would like to know how to find what my final drive raito would be with a 700R4 trans and a Dana 18 tranfer case with a warn overdrive. How would the two overdrives work together and would this let me run lower gears in the difs.  

the overdrive in the 700 r4 is approximately .70 to 1

the high range in the model 18 is 1 to 1

and the overdrive in overdrive is approximately .75to 1

http://www.advanceadapters.com/acrobat/saturn.pdf

take all of these and multiply them

.70 x 1 x .75 = .525 to 1 (remember these are estimates)

With the .525 to 1 final ratio you could probably run lower gears

but you are also going to in turn  have higher gearing if you add bigger tires. Imagine a bike, the bigger the wheel the harder it is to turn, but the faster it will go once it is going.  The benefit of all the overdrives is that you can use the low first gears of the 700r4 and the low axle gears to get the bigger tires turning, and then use the multiple overdrives and bigger tires to cruise with ease on the highway.  just make sure your engine stays in it's powerband, and that the tires and gears are all matched to that point.

what motor, axle gears, and tires are you planning on running?  I assume this is for a jeep, what year and model? 

I am planing on running a SB 350. I have not pick what tranny that I would like to use yet. I had thougt about the 700R4 but did not know it would work with my DANA 18 that I have and it has the warn overdrive in it. the axles are DANA 44 out of a 1976 jeep wagoner and all of this is going into a 1953 willys wagon.