Unless you are attempting to restore an original car that came with a side oiler to period correct specs it probably has little value. Even the original cars did not use a 427 side oiler in every vehicle
From a horsepower perspective it adds nothing. From an engine longevity perspective on a high banked NASCAR oval there can be an argument for them. Nearly 60 year old block castings are a bad bet for reliability.
Instead get yourself a new 427 iron block from someone like BBM that uses modern casting technology. You will have an excellent foundation for more hp than you will possibly be safe with, using a durable modern and new block casting. BTW the BBM cast iron block is done as a side oiler. Click here =>
BBM 427 cast Iron Side Oiler.