I need the goods on this topic.
In another thread here there was a clearance issue withthe exhaust and the original motor mount crossmember located under the firewall and the advise was not to mess with it.
On my frame this has been removed and is running a boxed Mustang ll IFS with a support installed to hold the T5 tranny.
Looking at other modified frames on this site and new repo frames this crossmember has been removed or not installed.
I guess my question is it okay to leave this out?
IMHO, in your case, you are fine. The MII crossmember and boxing takes the place structurally of the original crossmember removed. You've changed the load dynamics to that of a modern vehicle, and you should have no troubles. I'll also add that in your pic, it looks like yours is done better than most. Nice job.
The original crossmember counteracts the rotational forces applied to the frame by the front cab mounts. It was by design not chance that this crossmember was placed in line with the front cab mounts. If you can stand on the cab mount "wing" and there is no flex in the frame at that point then you should be OK.