When installing a 250 in a 67/68 using 69/70 frame brackets the problem is the unavailable rubber mount with the offset stud.
The 69/70 cars have the same problem when their rubber mount wears out.
The 69/70 engine frame bracket is narrower to accommodate the 250s wider block and its shorter, thats why the 69/70 uses the rubber mount with the offset stud to properly position the block fore and aft.
If you use a 65/66 or 67/68 rubber mount on a 69/70 frame bracket, the engine will sit to close to the firewall. The transmission will not line up with the cross member, the driveshaft will be to long and the shift linkage will not line up.
Until the 69/70 rubber mount is reproduced, I think the best solution is to modify the 69/70 3-bolt engine bracket to accept the 67/68 rubber mount. All that would take is a piece of flat steel welded over the 3-bolt bracket, then new mounting holes drilled.
I did some eye balling tonight on the mounts. Tomorrow I will get some pics, it actually looks pretty easy to mod the 67/68 rubber mount to fit the 69/70 bracket.