no more than .035
( .044 inch ) according to motorcraft.com
A 1968 Ford Galaxie 500 with a 390 engine should use a spark plug such as an E3 brand spark plug. This plug should have a tapered seat with 18mm threads.
fordparts.com shows .035 inch for two spark plug numbers and .030 inch for another spark plug number ( I believe the .030 inch is for heavy duty use )
According to one of the Ford websites : For a ( 1960 ) F-100 , 3.6 liter / 223 cubic inch six cylinder engine : ( .035 inch spark plug gap )
According to one of Fords websites : For a 1963 Ford F-100 with the 4.8 liter / 292 cubic inch eight cylinder engine : ( .030 inch spark plug gap )
gap is 0.035"
.035 inch
.035 inch ( according to ford parts . com , no spaces )
According to one of Fords websites : For a 1967 Ford F-100 , 352 cubic inch V8 engine : ( the spark plugs are gapped at .035 inch )
.035 inch ( light duty ) .030 inch ( medium duty ) according to fordparts.com
no plug on trans pan----u have to drop the pan
.032 - .038 if it's a stock motor.