LD4B or LD340

The LD4B had smaller runners to match 273/318 heads (D4B was similar with pre-66 bolt angle). LD340 had larger runners for the 340/360 heads. Much like the Performer/Performer of RPM today. All are good intakes for their intended purpose (not that the factory iron intake was at all bad).

I'm using an LD4B on my 318 build for 2 reasons. First I have one gathering dust, and second I will absolutely not suffer the sideways-a$$ed thermostat of a Performer. I'm also only building a 300 HP street engine. If I was building a 340/360 or more than a 300 HP 318 I wouldn't screw around with small port heads and would probably just get myself an RPM.

Really 300 HP is about all small runner heads support. Beyond that, you'll be much happier with the bigger heads and intake, all else (compression, carb and exhaust) being equal. If you plan on going much beyond 400 HP, consider a single plane.