I would like to ask a few questions, on related to your timing problems but could have an answer to your no compression problem. You switched from a solid lifter cam( stock for 273) with adjustable rockers to a hydraulic lifter cam and used the the adjustable rockers, am I right at this assumption. If I am, are you using the 273 solid lifter push rods. If you are the push rod are to long and they are holding the valves open, there for no compression. When switching from solid to hydraulic lifter you need to put in a shorter pushrod. Solid lifter push rods for a 273 should be 7.5 inches long, the push rods for the hydraulic lifters with adjustable rockers are around 7.375 long.
If you switched to the stamped steal rockers and push rods for the hydraulic lifters, then every thing is good, if not get the correct cup and ball shorter shorter push rods.