440 rods in 400

I'd stick with the 400 rods in the 400 block with correct crank and pistons. The pistons could be off the shelf ones which would be cheaper than custom ones with a raised pin. As far as 87 octane gas goes you want to keep the static compression ratio to 8.5 or less. The intake system (including cam) will control what your cylinder pressures will be...which will ultimately define what octane you have to run as well as the altitude.