just finished a lower I had bought a few years ago.  it is a delaware machinery lower.  All went well with the build but went to install the upper and the lower receiver appears to be just a tad too tight and will not allow the upper to fully seat to install rear take down pin. basically the area on the upper that has the hole for the take down pin will not slide down into the lower.   I tried another upper and it had the same issue so I am assuming the lower is slightly out of spec.  Should i take a dremel to the inside of the lower to allow for proper fitment?  

Be very careful.  Better to remove too little the first couple of them than to go too  far the first pass.  0.001 on each side could easily be the difference between the upper fitting or not.  If you have access, a mill is the best tool.  A good carbide bur in a drill press with a decent cross slide (X/Y) table would suffice, but patience and a needle file would get it done too.  Probably all with less chance of screwing it up than a dremel.  But if you have the patience and a variable speed dremel, and lots of experience/practice it would work as well.  

FWIW, the part of the upper that fits down inside of the lower is referred to ad the "lug".  Usually differentiated the same as the pins that go through them, front being pivot, rear being take-down.

I just did this myself. I used a rotary file in a drill rather than a  Dremel.
If you use a Dremel use a slow speed.

thanks for the help and replies! i will try a file first and see if i can get it to work.

jeff:  Please clarify. Maybe some pictures will help. Is the rear edge of the upper catching on the buffer?  If so, just push the buffer back each time you close things up.  If the receivers themselves will not fit together, you may have to carefully remove some metal.  Better to modify the upper than the lower in case that is necessary. - CW

I have one of those lowers that has a PSA upper that fits fine BUT I have 2 other uppers I have tried on the lower and they have the same issue you have.
I simply leave the PSA upper on it and go on with life.
I always assumed the lower was a bit out of spec.