I used Sam's plenum on my '8 with a 390. Plenum fit after I split it, put a joint in the center (fore/aft), and added a bump (about 1/2" high) to clear the spider.
I've had very good results speed and cooling. I used a larger oil cooler mounted on the firewall and a 4" duct feeding it off #3.
N299AD RV8 QB, IO-390X, BA prop...SOLD
N199AD One Design...SOLD
N99AD BD4, flew for 22 years...SOLD
EAA Tech Counselor/Flight Advisor
CFII, MEI, CFIG