"Sagging Motor Mounts"

New to the forum. Working on a 84 C10 engine swap. Replacing the 305 with a '79 350 and having issues with the motor mounts. Each side bolted up fine and don't see anything that looks wrong. The problem occurs once the lift is unhooked that is when the engine sags so far it drops into the firewall. There is PLENTY of clearance when the lift is holding the weight, but the Motor mounts are not and it's dropping significantly and pushing the HEI into the firewall. Don't have the Trans installed yet and maybe that could be the issue, but I've pulled several transmissions and never had the engine fall backwards that much. The NEW rubber mounts are flexing (hard) and seems like excessive play. The through bolts are torqued down (before removing the chains). Am I missing something simple? Probably. Could it be the gap between the top of Motor Mount and Engine Bracket? The Brackets have not changed, reusing the original 84 brackets, only the mounts are new and they measure up with the old mounts. Picture attached of the Passenger side (front view & rear view)...appreciate any thoughts. Thanks.

Welcome from Austria. Driver side sits not the same way as the passenger side. I used poly inserts and these are directional with the tabs being different. Don't know if that's the case with the original clamshells.

put the trans in and then the motor wont tilt back..

Yep the transmission keeps the engine from tilting.Support the back of the block before you hurt your dustributor