Dave,

An 8411 servo is .73"W x 1.52"L x 1.32"H whereas an 8611 is .82"W x 1.58"L x 1.56"H which makes it .09" wider, .06" longer, and .24" taller. The pocket in the B-ARF wing is big enough to accomodate the 8611 but you'll have to compensate slightly for the difference in dimensions.

Refering to page 18 of the manual, when installing the servos on the maple blocks, ensure that you have a gap (shown in blue in the first photo below) between the end of the servo horn and the red line (which represents the inner surface of the upper wing skin). Go easy when you are sanding the angle on the maple blocks, you want to remove as little material as possible, particularly from the tall block. Your gap will be about .045" less than that in an 8411, so that shouldn't be much of a problem but if it is, you can use a piece of thin ply to shim up the block.

Refering to page 19 of the manual, you can see that the base of the servo is installed such that it is pretty much even with the outboard edge of the cutout (refer to second photo). Your 8611 servo is .24" higher and this difference can be accomodated by one of the following two ways:
a) install the servo 1/4" further outboard than shown; or
b) install the flap control horn 1/4" further inboard than normal (install it at 6-3/4" not the 7" dimension as shown on page 19)

It probably doesn't make much of a difference which one you use.

Regards,

Jim