For anyone who posted here trying to help, or anyone who may read this thread having similar issues, this problem was solved many months ago. I forgot about my thread, but I figure I should let everyone know what was wrong.
I ordered the Midwest Industries two-piece aluminum forend from Brownells. Before I ordered I emailed them asking if the forend would fit my DPMS LR-308 AP4. They said that it would. By the end of this thread I was so insanely frustrated that I took it to my local 'smith.
Turns out the forend is for a standard AR-15, which actually takes a very slightly longer carbine sized forend. This is my first actual problem with Brownells, but considering years of great service, I can't hold it against them. I've decided to keep the forend for a later AR build.