I do not know whether the Megacycle 480 cam is a replica of the Sifton 480, or their own profile to give the same valve lift, but I have run a dynamic simulation for a Sifton 480, using Prof. Blair's 4StHead software, and with a 42 mm steel intake valve, BSA type radius followers, and a single beehive (or conical, as you prefer) valve spring.
At 9000 rpm all was still functioning correctly with no valve bounce or component separation. Seated spring pressure was 125 lbs.
This software package not only computes the inertia of all the moving parts, but includes the flex of all the moving items of the valve train, and this DOES include camshaft flex.(Note JS). A Titanium valve would simply increase the safety factor, or add something to the rev limit if required.
The exhaust side is rather different because the valve has first to overcome the high in cylinder pressure before opening, and here the pushrod will be subject to serious loading, with considerable flexing as a result. In the flexed position, the pushrod behaves similar to a spring, and eventually it will revert or spring almost back to it's starting condition, and if in so doing it should catch up with the valve, it can push the valve to lift further than designed to do, and thereby ending up in a condition known as 'float', which will also include component separation.
This is a very short explanation of the valve train events, but it should serve to notify those interested that it is rather more than a guessing game to get all the performance ducks in line so that they behave well and in unison.
All the above is relevant only to the Sifton 480, I have no data for the Megacycle 480, so results with that profile could well be very different.
Re seated spring pressures, I can turn back to the last work I did with V8's, and a quick search revealed that valve lifts were 18 mm plus, and spring seated forces were 325 lbs. Drag racers are now lifting valves more than 25 mm, and tripple valve springs are used, but their saving grace is - they only have to last for an extremely short run, a matter of a few seconds, then the engine is quickly torn down and critical components replaced before the repeat run.
One final point re acotrel and his fascination for 2 into 1 exhaust systems. Alan, this discussion is basically concerning a high power Commando engine, and quite simply single exhaust pipes are the only way to go if max.power is the goal. Forget about two strokes, they were never ever championship contenders with coupled pipes. Despite the short power bands of the early Japanese motors, they were still winners, so the torque, however little it was, must have been adequate.