Thanks Guys for the Inputs.
I looked up the Tomahawk website and found that Ventral fins seem to be a recent addition because there seemed to be many more Vipers without the ventral fins.

I finally called Thomas and got the details. so for those who might be building the viper here are the measurements.
The ventral fins are 40mm from the rear of the fuse at the step where the steel tube at the rear ends.
The two fins are 90 mm apart at the root where they are glued to the fuse and 140mm apart at tips.
I was planning to put in a Wren 160 but I saw one fly with a 120 yesterday and have changed my mind now , the 160 will be tooo much for this airframe.
The performance is marginal with a P80 but just right with a 120.
Xantos