Craig - I am putting a 292 with a T5 tranny into a '51 suburban with a '67 chev pickup rearend. I have looked this over several times with other people who have done the same swap with a 250 and 292 and it is doable without much effort aside from the motor mounts.
If done with careful attention to motor position, it can even use a traditional fan on the waterpump -
As Fred mentioned, you can use your stock SM420 with a later model bell housing - however this limits how far you can set the motor back toward the firewall and may make use of a traditional fan on the waterpump impossible.
