you might want to go back and ask yourself whether you want a zoom for the range 70-500, or whether you want 70-200 and 500 only.
the reason is, you might be better off getting any one of a number of 300mm F4 lenses and the Pentax SMC-F 1.7x AF TC and stay with your 50-200.
that way you have the lightest combo possible.
I have 2 long options, as follows
SMC 300 F4 and SMC-F 1.7x AF TC
or
sigma APO 70-200F2.8 and sigma 1.4x and 2x TCs.
the first combo gives a little more reach, the second due to shorter focal length but overall same lens diameter is a little faster.
Additionally, the second combo is more flexible, and because it is an automatic aperture lens, allows the use of P-TTL flash.
but note, the first combo while using a prime almost 35 years old and a TC that is almost 20 years old can give stunning results.
K7 with SMC 300 F4 and 1.7x AF TC at 1/40th (that's right) hand held
*istD with SMC 300 F4 and 1.7x AF TC and flash
K10 D with sigma 70-200F2.8 and 2x TC