There is a reason the L28ET distributor has a larger diameter cap, which is to allow a wider range of spark advance. I don't think you can reliably get that with a locked N/A distributor.
In regard to the SVO injectors, they flow 370cc/min, which does support about 300hp. I dynoed an altitude-adjusted 313hp to the wheels with those injectors, but for reliability I think you should use a higher flow unit for 300-350hp.