I put my engine back in the boat three weeks ago. I had it out for some minor mods, oil pan oil cooler. It ran perfectly last year so I couldn't figure out why it wouldn't start when I put it back. It cranked very slow, and chugged once in a while like it wanted to start, but wouldn't quite catch, the starter smoked, the battery terminals melted, etc. Sound familiar? First I thought it was fuel, and spent hours chasing that issue. Then... maybe I don't have spark? Verified spark that's not the problem. Then I thought it was batteries, cause the cable had gotten hot enough to melt the terminals. Got new batteries, still nothing but a slow turning engine. I tried the charger, and boost charge, etc, etc, blah, blah, blah. I spent a good 16 hours in my driveway chasing ghosts on this thing (plus I pissed away $100 on two new batteries, now I have 4). My engine was acting exactly like yours. Finally, I decide to recheck my timing. Crank the engine up to TDC, pull the distributor, and reinstall at 0 deg advance. Crank the engine and she fires immediately!
If you don't do anything else, go back to square one and redo anything timing related. Don't just look at it cause it's to easy to miss something. Talk to your builder about the cam timing. Pull the distributor, wires, etc, and redo the timing. I can't emphasize enough how much my engine was acting like yours, and it was a simple timing issue.
Good luck man!