I agree with Dave, and would be very wary about having totally solid mounts anyway because of engine harmonics being compromised and the transfer of any vibrations into the chassis.
I know that you are not following the original 917 set-up totally, but i would urge you to be carefull of any extra tubing and weight that you are putting into the rear end of the car.
A lot of experience with these cars, and the less like a pendulum you can make it now will pay dividends when it comes to set-up and handling. It is one helluva large rear deck going on top also, and with such a forward driving position you will know that rear end is trying to throw you off !!!!! Normal road speeds are not going to give you that much downforce that the race cars achieved and helped keep them planted.
You are doing a fantastic job and i know it is easy for any of us to throw comments about.
I realise yours is probably going to go on the road and will never be pushed as hard as one would in racing, but i would try to brace the bulkhead/main chassis rails somewhat, as they do, in the event of a shunt want to break in half at that weaker point due to the main weight of engine/gearbox, etc, and also if you are going to put the fuel tank transversally behind the bulkhead do double skin it, unlike the originals. In fact the more gussets you can put in, the less flexing there will be in the chassis anyway. Don,t rely on the bodywork bonding to stiffen it totally.
Sorry, don,t want to tell "grannie how to suck eggs" but these types of 917 set up do have their weak points and peculiar characteristics and i don,t want you to fall foul of the SVA/IVA examiners.