I don't know how much work you have done to assess the counterweighting on that crank?, So forgive me if this is treading familiar ground;-
That crank has very wide cheeks, and not the biggest counterweights either, the engine I am guessing has very heavy pistons too? 400g+?
it may well be very undecounterweighted because of space/production limitations, but it is common practice to use balance shafts to counteract primary forces of axial unbalance as well as secondary forces of unbalance.
Take a look at the 2 honda K20 cranks
this first from the K20A
this second from a K20Z4
you can see the newer engine has fewer counterweights -
honda have done this to reduce the very considerable weight of the k20 crankset to improve responsiveness - obviously, but they have offset the [primary] axial inertia forces with the use of the balance shafts, which ALSO act against the secondaries caused by the very heavy honda piston mass. This is a costly [ balance shafts obviously are a big cost add on in a production engine] but has the advantage of making the K20As successor a much much smoother but also freer revving engine. Trouble is if you remove the shafts, not only will you reintroduce the vibration that happens at a frequancy twice that of the engine speed, but you introduce primary vibration that will cause the engine to bounce - unless of course you fit very much lighter pistons.
So looking at that set up I think you are right to want as much stiffness in the block as you can get, I am not sure the balance shaft frame is going to contribute much, but replacing the caps with an integrated sump/crank carrier would be a very neat solution. There are the machining issues to consider but they are not insurmountable. Millington made a casting like this for their 2L, and it is common practice in contemporary F1 where of course undercountereweighted cranks are the norm - generally 30%.
As you know I am not an advocate for the absolute necessity of running ideally counterweighted cranks in a track engine or road engine, there has been a great deal posted on seloc that is misleading on the subject concerning K series, and some of the early empirical work done to modify these cranks was badly flawed, but hanging onto the crank is the most important thing and the further you move away from ideal counterweighting the more stiffness you need, plus at 7750rpm you are using some revs too.
Anyway maybe this is all well trodden ground for you, but if not I hope it helps
