You may as well use the rules to your advantage and go much bigger with your tyre sizes. You can easily get a 265 in the back of a SW20 even in Clubsprint, so go further because you can and run the 295 you're allowed to on the rear and go even bigger on the front as well.
 
If you were trying to build a car for Clubsprint these sorts of things would not be on the table. But as you're going for Open class i would be taking everything to the extreme of the rules rather than "this will do"
 
You're correct with the weight factor, has been some time since i perused the rulebook.
 
A arm would be awesome but is sooooo much work and the gains although there would be better spent elsewhere (all the fastest MR2's in the world still use macpherson strut front and rear)
 
Goodluck with the build and keep posting updates.
 
 

No need for tubs on the rear for 265/18's on stock body. 

Try driving them before you dismiss them, theory is great... "in theory"

Have a look in the 'race and rally' threads. There is some useful info on building a track car.
There are a few guys in Sydney who can build the cage for around 2k, more if you are looking for something a little more advanced.
I would also suggest using a hard top shell as there is less work involved and a handy weight save over the T top plus leakage issues.
If you are keen on running huge tyres both height and width, you will have issues with wheel bearings. It will also suffer too much scrub. To run these big wheels it would be best to redesign the suspension linkages to push the hub out the a optimum location as the standard location is simply to narrow. Axles can be fabricated to suit a new 'track'. Adding huge offsets is a waste of time.
Another point is the need for super wide/big wheels. The Lotus in Prod Sports circulates a 59 second lap at Wakefield on 205/16 and 240/17 tyres/ 380 rwhp/ 750 kg. A SW20 NA with 205/16, 225/16/90rwkw/1250kg runs a 1.10 at Wakefield. You do not need enormous tyres to be quick. Most people move to 18's for better tyre management for longer races plus availability. The Lotus has a superior suspension system which give its the advantage over the MR2. a MR2 with Lotus suspension would be a lot quicker than they are.
I checked the reg's for WTAC. There is no allowance for a V6 (which is ridiculous) but you could talk with management once the car is ready.
You can always look at being involved with MR2 Challenge while you are getting up to speed or part f a 6 hour regularity team. I am always looking for help at the Challenge and will need it at the Aust Endurance Championship in 2016.
 
 

So I bought a thing today. Still stripping out some sections of the car to ready it for a cage but I thought I'd post up some progress updates. Genuine Sparco Pro2000 seat with sparco sliders and mounts.

I will in future but not until I have a carbon replacement.