And while you're at it, throw some c4 in the oil pan and make it a good explosion. Ok, so that may be a little bit dramatic, but you have the engine apart, build it right. You can't afford to do it right the first time, but you can afford to do it twice right? With taper in the bore, the rings are constantly expanding and contracting as they go up and down the bores. IF there's a groove at the top, you could possibly break rings when the pistons come to TDC. Do yourself a favor and do it right the first time. Piston slap is likely the least of the problems as you have a tall piston, but ring seal will be compromised and on top of that, if you get some new pistons, then you can get the compression up where it belongs and make some real power. Just a suggestion, but it's what I would do.