For the Bootable USB you should use GPT Partition Scheme for UEFI BIOS and NTFS as the file format. More details here:

http://dellwindowsreinstallationguide.com/download-dell-windows-7-64-bit-reinstallation-sp2-oem-iso-for-a-uefi-bios/#BootableUSB 

For Windows 7 64 Bit:

• Secure Boot Disabled
• Legacy Boot Disabled
• Legacy ROMs Enabled

More details here:

http://dellwindowsreinstallationguide.com/uefi/#EnableSecureBoot 

http://dellwindowsreinstallationguide.com/a-clean-install-of-windows-7/windows-7-installation/ 

Thanks Phillip. So it is OK to create the bootable USB from the unofficial 6th gen Skylake iso even though my system is a 4th gen Haswell?

Also, my current (factory) Win 7 Pro install is MBR partition with a legacy boot enabled.  I guess that doesn't matter since a clean install will wipe everything anyway.  Is there any reason to go ahead & enable UEFI boot on my current system?  Will it even boot if I don't change anything else?

Thanks Phillip. So it is OK to create the bootable USB from the unofficial 6th gen Skylake iso even though my system is a 4th gen Haswell? Also, my current (factory) Win 7 Pro install is MBR partition with a legacy boot enabled.  I guess that doesn't matter since a clean install will wipe everything anyway.  Is there any reason to go ahead & enable UEFI boot on my current system?  Will it even boot if I don't change anything else?

Yes the .iso will work with your system (it has updated drivers for 6th, 5th, 4th, 3rd, 2nd, 1st generation systems). The GPT partition scheme is more stable than the MBR partition scheme. GPT is used with a UEFI Boot is improved. 

Thanks a lot.  Two more questions:  1)In case of HDD failure, can I use my current system image of (OS)C: which is on the MBR partitioned HDD  after a clean install of the GPT partitioned Skylake iso on a new HDD?   2)My wife's laptop is a Latitude E6540 with a fourth gen i-5 4300 running Win 7 Pro 64 bit, Simbios 2.7.  So I can use the same Skylake bootable USB for her system as well?  Sweet!

Thanks a lot.  Two more questions:  1)In case of HDD failure, can I use my current system image of (OS)C: which is on the MBR partitioned HDD  after a clean install of the GPT partitioned Skylake iso on a new HDD?   2)My wife's laptop is a Latitude E6540 with a fourth gen i-5 4300 running Win 7 Pro 64 bit, Simbios 2.7.  So I can use the same Skylake bootable USB for her system as well?  Sweet!

I've used the Skylake .iso on 1st-6th generation Intel Dell systems which have Windows 7 Pro, Windows 8.x Pro or Windows 10 Pro OEM licenses.

I don't know why you want to use a MBR image after a clean installation using the GPT partition scheme (but yes you could revert to it should you change the boot options back to legacy).

Thanks.  My current HDD partition is MBR.  The only backup system image that I have with all my programs & data is of this drive.  So my question was related to restoring my programs & data in case of HDD failure.  I was asking if I clean installed the Skylake iso with a GPT partition on my machine with a new HDD could I then restore my programs & data from my current System image & keep the GPT partition.  So, it sounds like the answer is no, & I will have to manually reinstall all my programs if I want to keep the GPT partition.