I used 300 degrees for about a half hour. Once you have the head hot, you can use a little spot heat around the guide from a map torch to keep it hot. You may need to reheat in the oven before you get all the guides out. Put the head back in the oven before you install the new guides. Put the new guides in the freezer for a few hours and smear a little grease on them to install. I used a steel drift to drive them out but an aluminum one to install. You don't want to distort the new guide if possible. It's a good idea to have the new guides reamed to proper clearance on the valve stems when you send the head out to have the seats recut.
Also, make sure the part of the old guide in the port is completely free of carbon before you drive it out or it will destroy the interference fit between the guide and head.
Htown16, Houston, Texas
1978 T140E Bonneville, 2004 XL 1200R Sportster, 1972 Commando,
1974 Commando, 1970 BSA Thunderbolt
Everything will be alright in the end.
If its not alright, its not the end.