273 questions...a few of them

what I do not understand is on the 273 in the D-Darts they used a I-495/E-505 lift cam in it? how can your 460 not work?

the retainer locks sank the retainer and spring down on the exhaust valves as seen in pictures. That will not change the depth of the valve (it's still on the same seat and height) only the point of bind in the spring and possibly tension down deep at lift. are the new retainers hitting the tops of the guides and binding there?

I saw the pistons have double valve reliefs so even if the piston was in the wrong way it would still be ok. In fact thats an old trick to get rid of the piston offset and worth a few tenths. but they are facing notch forward as they should be.

Is the chain and sprockets new? perhaps one is manufactured and it is marked wrong? compare the mark locations if you have another...

component speaking there are only a few things to create this issue. and it appears most have been covered.