The return line for the 258 is already on the driver's side. You just need to use a rubber fuel line to connect the return fitting on the fuel filter to the hard line already running on the frame rail. You may have to bend the hard line a little to clear the exhaust.
On the two V8 conversions that I have done, I used a piece of steel line and routed it from the existing 258 line on the frame rail around the front, under the radiator and back up by the power steering lines. Then used a short piece of rubber fuel line to go to the fuel pump. You could also replace the supply line from the fuel tank with a V8 driver's side line. That would be more work, but a cleaner install.
Jeep #1 = 76 CJ5, 304, T18, Twinstick Dana 20, Lock-Right in front, Yukon HD front axles, One piece axles - Detroit Locker in rear, 2.5" Rancho Springs, 1" lift shackles, 32x11.5 Super Swamper TSL/SX, Warn XD9000i, York OBA.
Jeep #2 = 80 CJ7, 401, T18, D300, 3" Black Diamond Lift, TrXus M/T 33x12.5, Yukon 1 piece axles, Auburn posi in rear, Detroit TrueTrac in front, Ramsey 8000.