You may be able to go .020" more on your bores, but it depends on core shift. Sonic testing is the best way to determine how far you can bore. A sleeve in the bad cylinder could save your current rotating assembly if there is just a problem with one cylinder wall. It would be cheaper then balancing a new rotating assembly and buying new pistons and rings for an overbore.
The stage one valves will have more shrouding in a 430 block. I would use a 455 to have the least shrouding with stage one valves. The early 400/430 heads were prone to cracking. I would want to have them tested for cracks before spending on a stage one valve conversion.
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