Rick's (olddog's) suggestion about a cage or mounting cradle is a very good idea. When Ford did their own IRS packages they used this approach. This is what the OEM IRS cradle for an SVT Cobra with IRS look like;
You can see the four attaching points at the top and bottom and left and right in the picture. There is a lot of "stuff" in the OEM implementation that we do not need. A company called Ronin Speedworks offers kits to marry the Ford 8.8 IRS up to a variety of performance cars.
Ronin likes the Ford Explorer version of the IRS housing for its added strength (so do I) but their mounting approach works with either the Explorer or Cobra versions of the 8.8 center section.
Here are a few pics of one of their kits. They have several other and I suspect would be willing to work with you if you wanted something out of the ordinary. This kit is particularly nice because it provides for a simple vertical install and retention of the IRS center:
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This rear mount style gets away from the ever present danger of cracking the thin rear cover on most of the Ford offerings. The cover in the picture is from an Explorer and is substantially stronger than the OEM Cobra style rear cover. Their rear mount style allows the center to be installed vertically and easily secured with two vertical fasteners.
This is a view of a front mount package to address the pinion mount for the center section;
This cradle in the picture is mounted into an Explorer subframe. You would mount it into the SFP rear frame rails instead.
When you use this style of mounting the install is as close to bulletproof as you are likely to get. The real nice attribute when you are finished is the ease of installation and removal of the IRS center section. Four vertical bolts and it is out, making service much easier — not a snap but much easier.
If you have good fabrication skills the pics should give you some ideas for a start on building your own design.
Ed
p.s. While you have the whole shebang apart give some thought to changing out the passenger car IRS housing that SPF provided for the more robust Explorer SUV IRS version Ford built for the Explorer. If you do be sure to use the version with the two large front mounts not the one with the single large front mount.