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7,000 isn't an exssessive amount of rpm, a well built sprint engine should be reliable at those revs with nothing more than a carefull build, how old was the engine in terms of bearings crank grind before tha first blow up? was the engine properly rebuilt/cleaned/new oil pump afterwards.
have you been suffering oil surge? that can very quickly ruin a set of bearings, if at all possible build the engine using lead copper bearings, they are far more tollerant of a bit of oil surge, or forien bodies in the oil than std tin type bearings.
ideally when building an engine balence the crank, then add the flywheel balence that and so on with pulleys clutch covers etc, but that wont be the cause of your problems,
i did 6 seasons on an engine that was reved to 8,000+ unbalenced, i say unbalenced , the origonal engine was, but over the seasons, it had a crank swap, different pistons a couple of rods changed and the flywheel changed/ lightened 3 times, in the and it ran a bigend, but by then the bearings were two and a half seasons old!
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