In theory... for a dedicated target rig you'd be best going with a solid bottom and small ejection port i.e. solid top receiver for the sake of stiffness. If memory serves (I had an early TAC30, S/N x004 as my first and only custom action a few years back) the top isn't cut out *quite* as much as a standard Rem700 action... but if you're spending the $$$ for something purpose built and want to go with a round-body action, why not look at the Predator 1000? Granted, I've never had a problem (that I know of) with 30" barrels as big as 1.000" at the muzzle hanging off a Savage Target Action, but having a longer shank at the receiver end for more support isn't a bad thing...
If you do get a Predator, or specifically a TAC30, check to be sure you can get it with a smaller interference fit between the firing pin and the bolt head. AFAIK they use PTG bolts, and default to a large firing pin hole - compatible with pretty much any M700 action on the planet, i.e. biggest firing pin has to fit thru the smallest hole, and if you get a small firing pin and a large(r) hole like I did... it'll crater primers even on factory ammo, and hot F/TR handloads... yeah.
At the time, my only option was to spend the extra $$$ and send the bolt off to Gre-Tan to have the firing pin hole bushed. Simple, and not terribly expensive in the context of the entire rifle build, but still annoying that my brandy new semi-custom CNC machined action needed that in the first place. It always shot great, but once the firing pin hole was bushed you could just about stuff the case full of powder and never see a cratered primer. Not that you *should*, but its no longer the limiting factor.
I've been told that since then you can request a tighter fit version - PTG apparently started offering this after a while in response to customer feedback. Either way, best that you walk into this with your eyes open - just because its 'custom' doesn't mean its always 'perfect'.