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I am getting closer to completion on my Crown style project, and I have a question... I currently have polyurethane engine mounts installed, Clark's replacement rear mounts, and my diff is supported at the rear with struts and heim ends. Is this going to be an issue with the diff mounted solid? I also have a set of Moroso solid motor mounts, I'm just not sure what the best combination would be.
Thoughts?

Seems to me that the urethane mounts, since they have a tiny bit of give, would help your other mounting stuff live longer.
And it would help keep the engine and trans from being a structural member of the car - and you probably don't want them to be structural members.

Since there's no torque going through the motor mounts - just a bit of up/down force - you could probably run soft stock motor mounts and only notice a bit less harshness from motor to body.
Come to think of it, there's no fore-aft force, aside from the body pushing the motor package forward or dragging it to a stop.
(both of those forces go through the wheel bearings and trailing arms, right?)

cheers
Jay

The fact that you ask the question tells me that you are aware of the conventional wisdom on this ,, all solid or all rubber ,, the engine torques under load and that has to be transferred to the car somewhere ,, with the diff solid when the engine moves the force is transferred thru the transaxle to the rear mount which is solid ,, supposed to be a NO NO ,,, having said that I imagine most Crown conversions are similar to yours and this is first I have heard anyone mention it ,, shows you are thinking about it ,, my own car is not a Crown and is all rubber so I have no personal experience but I imagine that it must not be a big problem ,, note that in the Crown manual it calls for removing rubber from crossmember at middle location , says it is not stiff enough with rubber there ,, but also says a big block will break the trans case ,, only reason for that to happen is engine moving and trans taking the load ,, my feeling is you are probably okay but it is good you are thinking about it

I did modify my Clark's mounts by drilling several holes and inserting steel rods through them vertically. My thoughts at the time was that the rods would stiffen the stock mounts up and have less deflection. I guess time will tell, either they will hold up or fall out.