how do i work out carb sizes?

Stromberg 175s won't be too restrictive in outright flow terms, but they won't play nicely with a 300º cam at low rpm due to likely flow reversion.

That much cam needs one carb throat (or injection throttle body) per pot in order to get an acceptable idle a lower rpm/part throttle running. So really you need Dellorto / Weber sidedrafts (45s) or I suppose bike carbs would be a possibility though I don't know what ones to suggest.

Dependent on how you feel about technology, you could also consider injection. Again, ideally you want one throttle per pot rather than a plenum manifold and single throttle to give best idle /low end running. Again, options of side-draught style throttle bodies like Jenvey or something adapted from the bike world - plus ECU (plenty of choice these days) and high pressure fuel system, so a bit more complexity and cost to start with, but more flexible to set-up. Never under-estimate the cost of getting sidedraft carbs properly setup - especially if your engine is a bit unusual. Though I'd thing your engine is probably very familiar in the Scandinavian rallying world.

Nick