converting a 2 bolt block to a 4 bolt block isnt as simple as drilling and tapping new holes, it cost a crap load. the new caps themselves run around 2-300 bucks, the main webs have to be machined, then drilled and tapped which is another couple hundred, then the fun part line boring and honing which costs over or around 400 here. of course you need new bolts to...... so theres a grand and you havent even started to build the engine yet
in my opinion if you just switch to a good set of main cap studs and find or make a girdle you will have no trouble whatsoever.