Assuming your upgrade components are matched for each other, you will benefit most from the HSR42 after making the performance upgrades to the heads and valvetrain. It should at least be driveable in the meantime, but how driveable? I really don't know.
Will this be a daily driver??? You could always keep the stock carb on your Blast until you're ready to do the performance modifications.
FYI: the Mikuni HSR42 can flow 213 CFM, whereas the stock Keihin CV40 flows ~159 CFM, according to a document by "bwoltz": Carb Shoot-out