Hi,
I'm upgrading my 87 Camaro LG4 originaly carbed to a Holley TBI + Summit SUM-G3138 fuel pump (43gph , 85psi) and a Megasquirt-II ecu.
feed line is 3/8" which seems OK, but return line only 5/16
Should I change fuel return line to a lager one to avoid possible fuel system problems?

the stock TBI return line is also 5/16", but since you are running such a high pressure & volume fuel pump i think you could get in trouble with this. why don't you just use a smaller pump? 85psi and ~270lb/hr is just over-overkill!

in my opinion, you could easily run a stock delco TPI pump (acdelco EP241) which supplies you with about 50 psi and 160lb/hr which is probably enough to feed any TBI-driven 305 unless you are running some sort of super/turbocharger.
hth, ownor

I have modified my engine a bit. it has XE264 cam with apropriate new valvetrain. Ported stock heads, polished chambers, also milled down 0.04" (1 mm) to rise up compression. Edelbrock performer intake end full lengt hedders+ dual exhaust to sidepipes.
this setup should crank 270fhp. + little reserve fror nitrous.

OK.
today I have measured fuel presure between TB and return line. it is constant about 3.5 psi with engine not runing and fully opened presure regulator.
Seems I will not get in trouble.