I'm currently in the process of switching from factory carb to the Holley Sniper EFI. Since my '84 TA did not have the EFI tank, I purchased a new unit for an '89 5.7L and the sending unit to install the intake fuel pump. I also purchased the Walbro pump kit 5CA400HP (255 LTR) and 20' of Earls 3/8” Vapor Guard EFI hose & clamps. I will be using the existing 3/8" stock hard line (original feed line) for the return. My concern is with the sending unit return line is only 5/16”. I have heard that the return line should be an equal size as the inlet or there would/could be pressure buildup especially when the tank is full (since that return goes to the bottom of the tank).

After looking at the sending unit I see that there's a 3/8” line for the tank pressure valve (the one that is mounted above the axle). I was thinking of converting this with an added 3/8” hard tube to the bottom of the tank and using the smaller diameter vapor line (after removing the rollover check valve) for the pressure valve. This of course would mean that the vapor canister would need to be unused/removed and the 5/16” sending unit return line would need to be capped off. (Note: All emission hardware has been removed due to damaged & inoperable components and connectors over its 34 years of operation). This is not a daily driver and is will be used for fun & show. Your thoughts?