I welded a crossmember under/to my original frame. Unfortunately that was 10years ago and I didn't take pics or keep measurements.
BUT it will depend on what kind of mounts you use. The orignals have a "slant" to them but some aftermarket ones like I used on my 52 panel bolt to the block and then sit straight down on the mount that is bolted or welded to the chassis. You can see this kind by checking out them out in my photobucket below. If you send me and email I can send you a copy of the instructions for them that may give you the right idea.
I suggest checking out the web site for Chassis Engineering. I think they make a set of mounts for the 37 (surely the 356 to) that fit right in the chassis like my 52.
Hope this helps in some way.
As a precaution I might at that if you have a spare engine block and tranny shell, "set them in" to "mock up" the mounts before you do any welding so it'll be centered ect. I had to do that putting the 350 in my 37 pickup so that I did not mess with the firewall and still had plenty of room for the distributor.
EDIT: This link may help you
http://www.chassisengineeringinc.com/page11.html