Any type of oven is preferable to using a torch. I know that Mick Hemmings uses one, but unless you really know your way around a torch you are asking for it. The issue is slow and even heating. You can possibly warp the things. You might want to leave the valve guides to a shop you trust anyway. There's a chance that an oversize might be needed and you won't be able to tell if you don't have the necessary tools to find out. That and the fact that the seats are probably going to need cutting afterward makes it a more straightforward job if you let the shop handle it from start to finish.