Due to the high pressure used, line size on a SFI system is not as critical as it is with a carburetor. I've seen late model 600-700hp street cars with stock 3/8" line. I'm pretty sure a 5/16" line would feed your 3800 ok. Wasn't the original application for your V6 a return system? It would have probably been 3/8" feed and 5/16" return. What are you planning for return? A lot of retro-fitted fuel injection systems use a new 3/8" feed and use the smaller original line for return.
77 Vega Estate wagon- project(someday)will have TPI305/T-5, S-10 spindles/axles
72 Vega HB Drag Car -383/'Glide/9"(email@example.com)(firstname.lastname@example.org 1/8th)
76 Vega GT- 400/4spd/9" retired from active duty(rusty)
03 Silverado 2WD ex.cab (work truck/parts hauler)
04 Silverado 2WD ex.cab (retired)
06 Haulin' 20ft enclosed car transporter
93 Suburban- 350/4L60(retired)
00 Saturn SW2- wife's car
01 Saturn SC2- younger son's project
00 Saturn SL2- daughter's car
98 Saturn SW2- spare
and 3 more Saturn "parts cars"
Note- the very act of listing all of these has made me realize I have some kind of problem.....