Either way is easy: 3 options
You will have to decide which tank you want - either is easy to install. You will extend the tank attach flange during the spar assy process, and simply build a longer tank. BTW John Harmon can sell you 52 gal tanks if you want them.
I am not sure if the -8 tanks can be mated to the -4 style spar? I kinda doubt it, due to the Z-bracket requirement.
Then again, you could build 2 RV4 tanks for each side, and extend the tank attach flanges to fit this setup. This would call for some sort of transfer plumbing, or maybe a simple check valve. Best you use better filler caps on the inboard tanks if you go this gravity feed route.
Consideration: your max allowable gross weight will not change, so keep this in mind if your mission profile includes a back-seater and luggage.
Carry on!
Mark