Dave, remember you originally had two valves sticking (#1 & #4), so there may be too little clearance for any of the four. You might want to use some modeling clay stuck into the chambers to give you a print of how all four valves clear and align. Anything less than 1/16" radial clearance is a potential problem with an exhaust valve because having one side shrouded (and no valve rotators) will cause uneven valve head heating and warp.
A dremel could take you forever and a day to do the job, and you really need to do all four. An air grinder or Dumore with 1/4 shank carbide burrs and stones is normally used. If you are not experienced at doing this, cut a sheetmetal guard with the outline shape of one chamber plus the needed clearance to clamp on and protect the head's gasket face from boo-boos. If you make the sheetmetal carefully, you can use it with layout dye and a scribe to mark all four before you start. Keep an eye on how the gasket will fit, too. You do not want any chamber overhang when finished.
If you are the handy mechanical type, you could take an old exhaust valve, notch the side and braze in a carbide or piece of tool steel to act as a fly cutter. Sharpen it to give you about 1/16 side clearance. The tool could be used in a mill, drill press, or even on the block itself if you grind a flat on the shaft for a pulley/drive/set-screw. If you use a block, place 1/2" spacers between the head and block to clear your cutter and provide a peek at what is going on. Make sure your cutter design doesn't mess your block exhaust valve seats. This isn't any new trick, I used to fly cut SBC pistons in place for valve clearance like this. (no spacers) I don't know if your Riley is original or Yapp, but the Yapp heads have enough meat to cut clearance for even bigger exhaust valves.