IF a customer came to me with this plan, This is what I would recommend instead of a barrel block design from what I have seen.
Receiver, take a Bat Model M 8.5"x 1.55" receiver.
Make some custom Stainless Steel Pillars, I prefer 1" diameter recessed to 5/8" where they can be seen at the bottom of the stock. There are three main support bolts on a BAT receiver of this size.
barrel would be anything you wanted in a top quality barrel. Barrel length would be 30", 32" at max. The reason, you will not gain anything meaningful going longer and all you loose is barrel stiffness and add stress to the receiver and bedding system. I have tested my huge 338 AM with its 150 gr powder capacity. I have tested the same load in a 40", 36" and 32" barrel length. With the 265 gr AT RBBT, 145 gr of H50BMG produced 3545 fps in the 40" barrel, 3495 fps in the 36" barrel and 3460 fps in the 32" barrel. Now this is a chambering with nearly 50% larger powder capacity then the 338 Lapua Improved and the velocity loss is barely over 10 fps per inch of barrel loss from 40" to 32" in barrel length.
My point, this rifle will be made for extreme percision, the BAT receiver with a stainless steel pillar bedding system will easily handle a 30 to 32" 1.450" fluted barrel with nothing more in bedding.
Also, you can put it into a stock that is much more appropriate for this class of rifle, The BBR tooley 1000 yard BR stock is a perfect choice for this type of rifle. It has a higher CP then the 50LBR stock so its more comfortable to shoot, it is plenty large in the areas it needs to be for strength and consistancy, it rides just as well in the bags with a proper rear bag design and the trigger will drop down to where it should be in this stock, unlike the overly thick 50LBR stock that causes the trigger to hang down much less then it should.
If you decide to go with a carbon wrapped barrel, the need for a block of any kind is even less important as the barrel has alot less mass so the receiver supports the barrel much easier.
Only thing I would warn about the ABS barrels, they are hard as hell to get. They are expensive and it can be a fight just to get a barrel. I have two of them that will be on order for over a year come the first of the year with no sign of them showing up in spite of endless squeakin the wheel!!!
Plus the cost of their "big bore" carbon wrapped barrels is nearly $1100 from the last price list I saw. Even if the barrel life is 50% longer then an all steel barrel, which they are not, you could still get two all steel barrels for the same price as the ABS and have more rounds to shoot as well.
Just my opinion. If the barrels were easy to get and less expensive, they would be a steal, but as they are, I can not recommend them to customers at this time. Just to may headaches to get the barrels. They are great barrels and its hard to say I can not recommend them but I simply can not in good conscience to someone looking for a barrel.
So my recommendation would be and thats all it is, for what you want:
BAT Model M 8.5x1.55 single shot
BAT 20 or 40 moa rail, depends on what scope you want to use
30-32" 1.450" straight cylinder heavy fluted 1-10 barrel
McMillan MBR tooley 1000 yard BR stock
Custom Stainless steel pillar bedding system
Steel ADL trigger guard
Custom 4 port compact muzzle brake
With that weight barrel, I would say you could never tell the consistancy difference between it and a heavy V-Blocked rifle.
Plus you would be dealing with a 25 to 28 lb class rifle instread of a +60 lb rifle which believe me, even just taking from the truck to the bench makes alot of difference and a 25 to 28 lb rifle is plenty for shooting at any range. Built correctly, even an 18 lb class rifle can accurately shoot to 3000 yards(when chambered for the correct wildcat!!!
Again, not trying to talk you out of anything, just offering another opinion on what I have seen works best for this class of wildcat.