But in an '86 305, I wouldn't worry about the converter any more...you've got a hydraulic lifter cam (flat-tappet) and a set of regular, stamped rocker arms...both of which benefit from the ZDDP...
Those engines spec'd something along the lines of an SF or SG...and high ZDDP oil will meet that spec...
What cam is going in it? You can update the rockers with a roller rocker (less friction, and a 1.60:1 lift ratio instead of the stock 1.52:1 for a touch more lift at the same duration...) at the same time...just make sure that the cam is sized for vehicle weight, rear end ratio and the 305 displacement...
I ran the M1 10W30 of the time in my '86 T/A that had a 305...it worked great...had to pull the manifold at about 60K for a gasket leak and the lifter valley looked brand new...no varnish even...and there was no ring wear in the bore (no ridge at the top of the ring travel...).
I don't remember the part numbers, but the SBC will fit an oversize filter easily...how big depends on your manifolds/headers, but even in the T/A, I used the "truck" filter, which was 50% larger...