They are 35mm I believe. I use a shalacking (sp?) Compound when I put freeze plugs in. Sand the block to the clean the rust and stuff off, clean it with acetone, coat both the block and plug with shalack and let it dry then hammer the new plug in. Its kinda like contact cement and does a good job of sealign the hole well. Ive been running 15-20 on my l28 for almost 3 years now and never blown a freeze plug I dont think that has anythign to do with it, or at least it shouldnt.
The last motor i took apart with blown plugs had a blown headgasket and warped head from over heating