A well tuned 292 should get good gas mileage. My '64 C-30 got probably around 12-15. Fully loaded with a 5,000 Lb load didn't seem to make much of a difference. Maybe brought it down to 11-14. But I also believe it letting it breathe. I always ran a straight-thru muffler on it.
If you go with a Detroit and if you go with an Overdrive trans, you may not have to change the rear. May just work out just fine. I roughly checked the numbers and it looks like a 4-53T would weigh about 750 Lbs more than a 292-6. I believe some heavier Front Springs (maybe just add some Leafs) should handle it.
By the way, what is the GVW on your C-60?
