Need advice on installing new fuel line

My newly built 408 small block (67 Barracuda) has had problems at 4500to 5000 rpms. The engine gets off the line and runs great until I hit right around that rpm range. Frankly, that should be in its power range. (Eddie 800, Hughes 3237 cam). It seems to be cutting out from lack of gas. Tried bigger jets, different setup springs, changed the Carter fuel pump to a Holley high performance fuel pump.

So, thinking the old 5/16 line may be varnished or gunked up, I am putting in a new 3/8 line from the new tank to the fuel pump. It is a pre-bent line from Inline Tubes.

It looks pretty tight around the rear shock, gas tank, exhaust.

1) Will I be able to get it in there without removing a bunch of stuff? If I didn't have to put the line through a hole in the cross member, it might be easier, but I think that will have to come first.

2) Will the 5/16 sender be OK, or is a 3/8 sender necessary?

3) If I use the 3/8 sender, it has a vent, and sealing it with a vacuum cap it seems that the vacuum cap would rot pretty fast. What should be used to seal it?

Any advice on problems or solutions that others have had before I start would be a great staring point.

Thanks!