Heat is your number one problem when it comes to making sure you don't disturb the "hardness" of a bolt. If I were in your shoes, I would put a nut on the bolt, one on each end, then chuck it up in a vise and use a fine tooth hacksaw with lots of lubricant or water. Somthing that will keep the temp down when you are cutting. Just go slow and you won't have a problem. After it's cut, use a triangle file (or a die, whichever is easier) to fix the threads and then run the nut down the threads to make sure it good and stick it on the engine.
I will stake a claim. Many may dissagree with me........this is just the way I would do it. I am a regular redneck.
Joe