I believe you are talking about rocker arms not lifters?
Yes, going to a lower ratio rocker arm will decrease lift & duration.
You need BBC rockers or ones made for the 6 cylinders 1.75 ratio.
1.8 ratio.
The duration difference will not chage your idle that much & I do not believe running a SBC 1.5 ratio rocker arm will have correct rocker arm geometry. (I do not think they will work)
If it has 283 rods,then you are loosing oil pressure there because SBC rods are narrower on the big end therefore it will cause excessive oil slashing all over the crank & rods.
Did you decrease the hole outlet on the tube that directs oil to spray/drip on the cam gears? If not,that might bring up some oil pressure.
MBHD
