will the 273 solid lifters,pushrods,and rockers that you can ajust work in a 340, with a 500 lift solid cam ?

yes

Yep, that is what I used in my 340 with the MP 284/.528" cam with dual springs.

Usually it is OK but you need to make sure if they are the bar-bell lifters they do not go too far down in the lifter bore and allow the oil to flow directly out of the bottom of the bore. A lot of aftermarket cams have smaller base circles and add to that core shift and you can have a major internal oil leak.

If they are the solid body lifters than there is no problem

273 rocker arms used a bolt that is held in position by friction. I had to buy the new ones with a lock nut.

They had an interference fit on the threads. Mine were still tight, but I hear a lot of them were worn from adjusting them. You would have to retap and spot face for the nut. I'm not sure if the adjuster or the rocker had the oddball thread.