Which one is better? (long)

rbkt65 said
I am putting a 904 in it because i am not going to cut up the floor for the a500/518. I will be using a Walbro fuel pump hooked to a 3/8 sending unit. I have braided line to replace the stock 5/16 line. I won't be using any emission stuff. The 96 truck this came out of is a return system. I have both filler necks, 1 with nipple and 1 w/o nipple. Fuel filter will be inline type. I thought it might be easier to use the 4 nipple tank so that i would not have to piggyback a vent on top of the return line. I guess that if i use the filler neck with the nipple, I could put the "t" in the fuel return high enough to connect with the filler neck nipple.

The fuel neck vent does not get piggybacked into the fuel return, that would defeat it's venting ability and cause fuel delivery issues. The vent (when no charcoal cannister is used) is just open to atmosphere; from the neck it loops upward into the C pillar area and then follows the filler tube back down through the trunk, terminating inside the driver's side frame rail.
If the sending unit you intend to use also has a return fitting, then your tank will not require any additional nipples. If not, then your tank will require the extra nipple. There is no reason for a 4 nipple tank since you're not running a charcoal cannister or any other type of emission equipment, you will just end up capping them- more sources for future leaks.
Just make sure your system truly is a return style system, as DionR said. This affects your part selection and choices.
Your controller may need to be reflashed to ignore the lack of a transmission signal with a 904, and the lack of emission controls; which will generate fault signals and cause your CEL to be constantly on and possibly keep you in a "limp mode" all the time...
A 904 bellhousing will need to be cut to make clearance for your crank sensor.