Don't think you could, it is prety much at the limit of what the old blocks can take. The blocks are not lasting as it is!
I'm sure they don't - that was my point. The Ford Pinto was only designed for 100 bhp, and the 750 bhp Cosworth didn't last long, but it was used on the 1/4 mile where engines are usually stripped and rebuilt after only a handful of passes.
Seeing as anyone can play this game, this is my formula - two boxer blocks back-to-back, with a special billet crank and four 16V heads. This would make a 3.4 litre flat-8. Porsche put two 356 4-cam engines together to make the 908 in the same way (although they went one step further and cast a new block in magnesium alloy).
This type of engine would suffer less vibration than a 4-cylinder, so would have a higher rev limit, and would have an extremely flat torque curve. Quad 44mm Weber IDFs would be my preference, but of course there is always EFI and forced induction...
Basic out-of-the-box normally aspirated power, say around 280 bhp, before you start tweaking the camshafts and compression ratios.
An 8-cylinder boxer would be as long as an equivalent capacity V8. As I prefer mid-engined arrangements, I would use either the classic ZF transaxle or go for the Audi FWD transaxle, which has the benefit of inboard brakes. This is quite happy behind the Audi V8, so should handle the power of my hypothetical boxer-8 without complaint.
For the more conventional front engine layout, I propose the Porsche 944 transaxle, which is just the Audi FWD assembly in a different housing. Even with downdraft Webers the boxer-8 should provide an impressively low bonnet line. A nice grand touring coupe style would go nicely with this mechanical package - something like the Ferrari Pinin concept from 1980 (which just happened to have the flat-12 from the 512 BB mounted up front and RWD - now there's a coincidence
So that's my idea. I guess the icing on the cake would be a custom made aluminium cylinder block, but you can't have everything...
The enduring problem with Alfa Romeo in the last years before the FIAT takeover was that as a State-owned company they were unable to invest in the development of their cars. The legacy is therefore one of fast but fragile small saloons, not the best starting point for building serious horsepower. But things could be worse - look at FIAT for instance