JE & others will make you what ever pistons you want. Approx $100.00 each or a bit more.
As far as a V-8 piston,,,,it's been a while IIRC,you would use a 327 piston for a 4" bore for a 250 L6
People,, correct me if I'm incorrect.
Not sure why you would not want to use a V-8 piston?
SBC pistons are cheap.
What kind of compression are you looking to get?
W/a 4" bore, flat top & small chamber 194 head ,I would guess you can make 14:1 compression.
I had .030 307 forged flat top pistons w/4 valve reliefs & a 194 small chamber head & had 12:1 compression w/220 psi cranking compression.
Douglas, has a 4" bore 250 L6 in his race car,high compression,& it will over heat in about 5 minutes of driving on the street.
MBHD