Fixed the issue I believe ; I think it was a combination of things
1. A "sheath" for the main engine mount bolt didn't get installed during engine swap.
2 rear engine mount was missing washers and had bent the bolt that was holding it. Added thicker washers and replaced bolt.
3. Tightened chain to about a 1/4 of an inch in play. Adjusted till problem disappeared.
Two passengers and several WOT launches later no more lurching.