From what I have seen over the last 30 years or so:
Very few people outside of machine shops have the capability to replace guides, pull press-in studs and machine for and install screw-in studs, and grind and seat valves. In my shop, the labor for the stud work, guide replacement, and valve job would run around at least 400 bucks, less parts. Add to that valves, springs, studs, retainers, etc., etc., and you are getting close to the price of a set of far better heads.
Without these capabilities, saying that it is cost-effective to take a set of 305 heads and make them as capable as a good set of aftermarket heads is ignoring the fact that it costs money to do half the job.
Which brings me to the other half. Most guys haven't a clue as to how to or how much work is actually involved in properly and effectively doing the port and bowl work, and unshrouding, that is needed to make 305 heads work as desired. I have seen way too many botched or half-done port jobs that ruined the heads or had to be completed (at additional cost) to make the heads "work".
By all means, if one is has the equipment and ability to take on the task, go ahead. But you will still have, at best, a set of dated, thin-wall heads that are not what knowledgeable people would consider worth messing around with.