proper base for bff2

[SWAMPFOX]

i am wanting to mount bff2 on daughters 20ga. 870. upon investigation for weaver rail they indicate that rail only fits 12ga. and to use saddle mount which i dont like. can i still use 12 ga.mount here? can i use marlin 336 mount here? also what is proper distance to mount ff2 in relation to reciever and daughters eye? any thoughts, ideas , opinions are apprecited.

[SWAMPFOX]

got it figured out as i found that i had marlin 336 base. now i can not find my hardware. gonna pm warbird and burriss