how come no one makes a Big straight 6 like 400-500 cu inch, 400hp +.
seems like it would be easier to work on in a boat with a twin setup,
and reduced weight.

Hmmm... interesting thought. And it would sound like a bread truck. Seems that the old pickup trucks with straight six's did pull pretty good.

There was an old Mathews that I worked on in the 60's that had a big 6 cylinder (about the size of a vw bug) that had twin plugs in the top for each cylinder. Cant remember the brand name but it was in a very old 40 ft mathews, single screw.

What would be interesting is a pair of narrow angle v6's with screw chargers! I'll bet they could get to the 400hp range.

There used to be an engine builder years ago that made HUGE gas 6 cylinder engines for trucks... The name of the engine was " Hall-Scott"... They made small engines also for cars back in the late twenties. But... most people don't know that inline 6 cylinder engines of the same cubic inches as a V-8 weigh more... a lot more.
You have 7 main bearings instead of 5. The block and crankshaft are heavier. Instead of only 4 throws for the 8 cylinders.. Each cylinder has it's own crankshaft throw. The only advantage is width. Not too many people make speed equipment for inline 6 cylinder engines.

That is what "Torque" motors was trying to do with their 60 degree V-12... make a narrow, powerful, unsupercharged engine. But they had too many "issues"...

Any update on how callan did with these last race season?

I know they installed some ... I learned that from a magazine article... other than that... nada.....

You never heard of a Cummins or Caterpillar? I assure you the narrow-ness of an inline six is a benefit.

As far as a big gas motor, 4 liters is about the high end of the range for a straight six.
Hop up parts? Here you go:
http://www.cliffordperformance.com/

Since he didn't mention Diesel... I was assuming he was talking about gas engines. EVERYONE knows there are inline diesel engines.