Engines took me back to my childhood there:
I'm singing a higgity, haggity, hoggety, high
Pioneers, they never say die
A mile up the road there's a hidden cave
And we can watch those marginalized and oppressed indigenous peoples
Go galloping by
Anyway - assuming that the F-35A and F-35C make it past sequestration, hook issues, the IPP's flamethrower mode and certain contractors' apparent inability to locate their own body parts without a flashlight and a search warrant, the two-type approach could work and would give the RAF the stealth striker that they want.
As for range, I would pull the gun off the A (which should not be impossible with judicious use of B and C parts). Eliminates a lot of empty weight, frees up room for a lot of fuel, reduces drag and can't hurt the RCS, what with the present configuration being reminiscent of a donkey in Y-fronts. Also avoids introducing a unique new ammo type in the logistics system.