Short answer:
They will work just fine with BPEs. BPEs change the correct shock length anyway since they change the mounting points 1-1.5". If you bought the "correct length shocks", then wanted to upgrade to BPEs, technically your shocks would be too long.
Long answer:
To properly measure shock length, you really shouldn't just go by the lift height they are advertised for anyway. Yes, they will be close, but to get the maximum amount of travel, there are other factors besides lift height that play a big role. Tire size and bumpstops can change the optimal length shock very easily. Obviously, you can get more uptravel with say 4.5" and 31s than with 4.5" and 33s.
To do it right, and get the most useable shock travel for your $$$
1. Install the lift and tires. Also install BPEs if it's in your plans.
2. Go flex it out and measure for bumpstops (with no shocks/swaybars).
3. Install bumpstops, these are important!
4. Go flex it again and measure between the shock mounting locations on the
compressed side(s). You should be sitting on your newly installed bumpstops
5. Then find the longest travel shock that has a
compressed length about 1-1.5"
shorter than you measured.
IIRC Rancho has a nice chart for this, and most reputable companies will be glad to take your phone call.
You don't want to bottom out the shock, and rubber/poly bumpstops will compress more than you will be measuring. You want some give there, therefore the 1-1.5" shorter shock.
Hope this helps :dunno: