I don't think that the LA300 FEL will fit your B7100. At least without some modifications. Here is the information that I have.
For the B7100 HSD:
The B1630C is the Kubota model number for the FEL that is compatible with the mid-mower (reference: 1995-1997 vintage Kubota FEL sales brochure).
The B1630 (no "C" suffix) is the Kubota model for the FEL that is NOT compatible with the mid-mower (reference: 1995-1997 vintage Kubota B7100 tractor sales brochure).
From the same Kubota FEL sales brochure, as well as a newer one (2000 vintage?), there is no mention of an LA300. There is, however, an LA301 "Quick-Attach" loader that is designed for the B1700E/DT/HST and the B2100E/DT/HST.
Hope this helps,
Kelvin