Couple of problems here.
The PAMPA rules for Classic Stunt include:
"It is expected that contestant will comply with the spirit of the event and enter only qualifying models which as closely as possible accurately reflect the aerodynamic layout and appearance of the original. .... removable landing gear which allow the entrant to adapt to the contest site and conditions are allowable. This would not allow the substitution of a tricycle landing gear for a conventional gear, or vice versa."
The Shark has a tricycle landing gear - period. Unless there is proof that somebody flew a Shark prior to 1970 with a conventional gear, a Shark equipped with such a conventional landing gear should not be allowed to fly in the Classic event. Furthermore, unless there is proof that somebody flew a Shark with a retractable landing gear prior to 1970, a Shark equipped with such a retractable landing gear should not be allowed to fly in the Classic event. Using a retracting gear in a Shark may or may not improve its performance, but the "appearance of the original" in the air with the gear retracted would definitely be changed. (You might find an ED who thinks differently, but he would not be acting in "the spirit of the event".
Now, for another matter. As mentioned above, a retracting gear on the Shark may or may not improve its performance. There would definitely be some differences in the manner the model is trimmed to perform the stunt pattern. Would it look neat? Definitely! There should be no reason not to use a retractable nose wheel if you want to go ahead to build a Shark (or any model with a tricycle gear). There are light duty electric retract systems that has a nose wheel that rotates the strut as it retracts so that the wheel would be flat against the tank. These are available as a system from some of the RTF RC manufacturers. Besides, these systems are much lighter than anything from E-Flight and should be strong enough to work with a comparatively lighter model (like the Shark) than the RC models the E-Flight systems are built for.
The RSM Shark kit is a good kit. If you really want to build the Shark, fine. But if you want to build a really neat looking Classic ship with retracts, you might want to look at Harold Price's Crusader. It has a conventional gear with retracts. There is a photo somewhere of such a Crusader at last year's VSC.
Keith