Clodhopper, While you won't add a .223 barrel to the mix, you can enjoy your combo and look at the benefits it has. The barrels are lighter than present day barrels, and thus are really a joy to carry afield even on a full day outing. The 30-30 should probably have been drilled and tapped so a scope could be added. All the older H&R 30-30 barrels that I have had those real nice flip up sights from Lyman attached to them as well.
If you handload for 30-30 you can make up some real nice hard hitting loads using the lighter 125 and 130 grain bullets that exist today, or use the standard 150-170 grain factory offerings that usually showed good accuracy out of the Topper or combo rifle/shotgun.
And if you ever decide to sell/trade, the Topper crowd here would help you out!