I would recommend getting the flat 61 with a full frame or APS-c. I also had to get an M48 DLSR adaptor as the spacing is critical, You may need an additional spacer if you sony is mirrorless. Its a great little scope not sure id want to go any longer on the SA, but i haven't guided on it yet and if i was going to do it i may as well get the AZ-EQ6 out, saying that i have seen a few people getting good results using 400mm lenses and guiding the RA to limit the PEC.
Also worth mentioning that you can screw a 2" light pollution filter in the flattener and its got a nice built in rotator, everything screws together so no compression rings with is also a nice touch.