There's nothing wrong with using Grade 8 bolts, but the studs would be a better choice. Most studs have a coarse thread where they thread into the block, and fine threads at the end the nut fits. A hardened steel washer should be used between the nut and the main bearing cap and lube it with something like anti-sieze compound if you choose to go with studs.
The only time I'd reccomend using studs on main bearing caps would be on a high-output racing engine. Go with the new grade 8 bolts, and make sure they engage approximately the same number of threads in the block as the original bolts. You wont have any problems. Main bearing bolts are normally grade 8, so unless your originals are badly rusted or stretched, there's really no reason to replace them. I can usually tell by a bolt's "feel" when I'm torquing it if it's stretching excessively, but that's not a skill that can be taught over the internet. It takes a few thousand bolt-torquings to develop the feel, and even then it's not a sure thing, just a judgement call.
Jerry
