Russell, sounds like you got a really good trans. I really can't recall how the blocker rings are configured but I have replaced some & should have a couple laying around somewhere here.
I think the mainshaft nose pitting & related bearing wear is usually from mis-alignment. I never used to check it but after changing motors & using a proven bellhousing & trans combo & having grinding occur when shifting into reverse I now do. I found the back of my 292" block to be off-square from the crank centerline & the dowels to be off too. Wore out the crank pilot bushing real quick too. Fortunately that was with a 4-sp. Saginaw so the parts weren't too expensive. I have since seen that on some MY6 gear sets in core transmissions.
Can you post a link to where you got your parts?
Nice truck you've built. The 3.36 axle should be about right for this trans. I run a 3.08 on the sea level flatland here.- Doug

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