Check and compare Hooker Harness and Crow Enterprizes for five point harnesses. I personally prefer the cam-lock style release setup versus the military style but I don't do much aerobatics. Both companies offer this.
Hooker also offers a true aerobatic set with ratchet for a second seat (lap) harness but I understand these are difficult to fit into an RV-8 without some extra work. Be sure to check first.
My experience was first with Hooker harness (non-ratcheing type) but found they slipped a lot once G went negative even when just pushing the nose over from a full departure stall. If your wife flies with you and/or has weaker hands she might prefer the cam-lock release. You can mix and match, front and back. Hooker offers both release styles.
I currently use the Crow system and like it a lot.
Shoulder pads definitely recommended for either system.
RE: interiors. All are good suppliers. I've been impressed with Flightline on quality and response.
Chris