I'm assuming you're running a SM-420 4 speed, right? If the gears are grinding on an upshift, you need a new set of synchronizers. There's absolutely no reason to delay that long between gears other than a downshift to the non-synchronized 1st. gear which must be done from a full stop. I've never heard of taking several seconds to upshift a properly-working SM 420. Fix the transmission and you'll have a lot more luck avoiding lugging down even with that ridiculously high-geared rear end. Did you also change the gears in the front drive axle?
Jerry

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