The 4/6 top end will bolt right on to the 340 crankcase. If the top end is good, this would be your best bet. I would swap everything. Pipes, cylinders, heads, cdi, coils, clutches, gears, and carbs.
I don't have too much experience with radar runs, but I would be careful going too high on the gearing. Just because you have it geared high, doesn't mean the little 340 has the HP to take advantage of it!