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Whilst not knowing what cam and lifters (specific heights, not just the part number), this sounds like you have a couple different things going on here. First, changing the cam should never cause you to have blue oil smoke. The 258s are notorious for blow by. Let's do this first: Start it up, and pull off the PCV hoses form the valve cover. Do you see smoke/vapor, anything being chugged out of the open valve cover holes?
The valve train issue is most likely worn rockers and pivots. If you put a higher step cam (even a little bit higher), this stresses the rockers and pivots. You would do well to replace all of them. This too would not cause blue oil smoke. It is common for these engines to lose an oil control ring due to breakage (you would never see this on a compression test), or, the oil control rings are carbon seized. This is VERY common, as the combustion chamber , intake manifold and carb are very poorly designed. This allows carbon buildup in all kinds of places....in the intake, on top of the intake valves, and in the oil control rings. This really sounds like you are going to need to pull the pistons out.
It would also be my thinking that the break in oil combined with some carbon buildup on the oil rings caused them to seize up.
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Cam timing, no. Valve seals? Absolutely. If they are lifting, you are getting oil sucked down the intake valve stems. That would explain the instant blue smoke. I'd rethink the valve seals before anything else. Good point and call on that.
I'm pretty sure that umbrellas need to attach to the valve guide and stay put. They should not move. Either you have the wrong umbrellas or you need to machine the valve guide to accept and hold the seals in place. If the seals are being lifted up as the valve stem closes (raises), oil is being drawn down the valve guide. If you have a lot of blue smoke, you probably have worn valve guides too. If this engine was in my shop I would eb replacing the valve guides and machining them to hold the seals tight