Well, forget the pic for a starter, the barrel on the right is an NSR250mc21/28 barrel, the one on the left is an NSR150 one.
The VHM head looks like an RGV one to me, it's certainly not an NSR250 or NSR150 one.

The text implies these are overbored NSR250 barrels;
"Power Valve Manifold For Big Bore"
"Heads Machined to suit big bore"
etc.

Ther's no reason you can't overbore an NSR barrel to a convenient piston size indeed Eric Gorr did so for years.

But from my point of view one of the big attractions with the 300 kit is the fact it gives you new and obtainable barrels, if I was going to spend £600 tuning a 250, I'd spend £600 tuning a 250 not a 2xx.
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