I am rebuilding my 454 Chevy engine do I need to replace the old bolts or will the old bolts work properly. I understand that some bolts stretch and can not be reused.
If you are making the effort to do a high quality rebuild then I would fit new bolts for the head, camshaft/rocker carrier(s), main bearings and big ends and flywheel ie all the important ones.
If you are just going to "get it running" to sell on... then you may well consider reusing bolts, except the ones that stretch as you have recognized they need renewing anyway (although some ignore that anyway) ...
The big block Chevrolet (BBC) engines did not come with torque to yield (T2Y) bolts in them. As long as the bolts are in good shape (not bent or broken), they can be reused with confidence.
If you chose to replace any bolts, only the head bolts would be possible to do without further machine work. If you change out the rod bolts, you'd need to have the large end of the rods resized. The new bolts will place the cap in a different location, which would mean the bearings would not behave correctly and would require machining.
The same goes for the main bolts. If you went with new bolts or stud replacements, again, it would require it to get machined. This is typically called an align bore of the main caps.
Mind you, neither of these things are bad to have done. If you are looking for a very good rebuild job, something which will last you a long time as well as give you the best performance, installing ARP studs in the mains and new rod cap bolts will help the engine spin a lot higher and hold together better. All of it costs money, so you'll need to decide which is better for your situation. Really, though, as long as the engine was doing okay prior to it being torn apart, there should be no fear of reusing the old bolts.