What you're up to is referred to as a "Clevor". YES it's worth the trouble. The broken valve guide is no big deal at all, had one replaced on my Windsor Sr's for around $50. You will need to replace all the valve seats to run unleaded, but if you think about it with the exhaust valves being stuck, that's not a bad idea to do anyways. I'd also stick with the stock valve size, did you look at those ports???? You'd need a huffer up top to fill up those chamber with monster valves.
Here's a link to a thread talking about exactly what you're thinking about doing:
http://www.network54.com/Forum/threa...eid=1085978217
Hope this helps you out dude! Keep us posted!