i did not use the prebent lines as you have here, but looks like you got it there already. yes, you must remove that dual hole rubber plug. Get both lines to just start showing thru that oval hole, then that is when you thread them through. you gotta reach down and and force the lower one first, then the top one. It might help if someone is applying light forward pressure from the rear. Lube that chunk of rubber with PB blaster. After both lines are started through the chunk of rubber, you push that chunk back in with the lines installed. Careful, sharp edges, use gloves of some sort. Hindsight is 20-20 and I should have bought prebent ones. I installed mine from a coil of raw tubing and fed it in from the rear when the engine was in place. After the completed installation, I slit pieces of rubber hose and pushed them onto one of the lines so the lines would not rattle against one another and this put them under tension and has kept things rattle free. A great technique was threading a thick strand of copper wire through the OLD fuel lines from front to rear, then I pulled out the old line from the rear, then the new line was fed in on the is copper wire strand uncoiling stainless tubing toward the front.
Clay Perrine's excellent photos were my inspiration
http://www.914world.com/specs/tech_fuellines_ss.php