if it where I, i would be cutting the tube and up sizing the secondary straight up because you will be loosing a bucket load of the light cone though vignetting in the focuser draw tube, but that is just me.
http://www.iceinspace.com.au/forum/s...ad.php?t=59178
I go into quite a bit of depth on what i have done, and to be honest im actually thinking of moving to the next size up secondary which is the 88mm minor axis it would mean a 34% Center obstruction from the 70mm which is 27%, but well worth the extra light gathering capacity across the sensor! Andrews and Bintel both sell secondary mirrors at a pittance of cost (49 bucks plus 10 dollars P&H).
If you do decide the change of bolt length, be sure to pull the primary mirror out and drop some silicone blobs to hold the primary to the primary mirror cell and be sure to leave the mirror clips off the surface of the mirror.... they will cause big problems.!