A 16V would be a better base for a track car, mainly because the place you get the extra benefit from a Williams is on lumpy, twisty B-roads.
A track is smooth, so dropping it to the floor works well - but if youre doing that then youre best off getting a 16V, which costs a lot less to buy. Yes, the 2.0 bottom end on the Willy makes it more flexible, but youll be spending most of your time on the track at the sort of power band revs where a Valver is almost as quick as a Willy. Most of the suspension benefits of a Williams would be lost once you lower it and use it only on smooth tarmac too.
I dont think its a good idea to strip a Williams, but my main objection would be the cost. Youd get a dog-eared Valver for £1k, while an equivalent Williams would be nearer to £3k. For what you want, I dont think you can justify the extra £2k!