Daily Driver Engine 318 vs 360, what one?

Here's one that I built and got good mileage:

67-68 Cuda Fastback, 72 318 block, TRW 10.5:1 forged pistons (no longer available), stock cam from my bro's 78 Lil Red Express (stock 340 cam), Rhoades lifters, 77 360 heads (important to use the bigger cc - brings compression down to 9.2:1), stock late 60's 340 intake (square bore), Holley 600 vacuum secondary with electric choke and 10.5 power valve, dual snorkel air cleaner, good rebuild 904 trans, 7 1/4 2.76 axle, dual 2 1/4" exhaust with stock 68 exhaust manifolds. I got 17.75 MPG on the highway and probably just under 300 HP.

There were two issues that I had to work out. I originally built it with stock 318 heads and it would overheat when the temperature got above 80° F after 20 minutes of running. It would pull 24" Hg at idle. I found a guy with a set of stock 77 360 heads with intake for $150. I didn't use the intake as it was a spreadbore, but used the heads. The idle vacuum dropped to 22.5" Hg and the compression went to 9.2:1, that solved the overheating issue because the compression was too high with stock 318 heads.

The other issue: The 7 1/4 axle could not handle this combination. I broke 3 axles in 4 years. The last axle break, my left rear tire, drum, and axle came out and passed me on the shoulder of the highway as I was driving at 55 MPH!!! (I slowed down from 70 MPH when I heard the noises from the rear end before it broke). And the gear lube comiong out of the axle caught fire! I doused the fire with a shop towel from the car and had to track my tire with the drum and axle still attached to where it finally stopped rolling. As I waited for the tow truck on the side of the road, I decided to convert to a 8 3/4 axle - no problem after that (it had 3.23 sure grip which reduced mileage a bit). Luckily, I had super stock leaf springs with Mancini racing u-bolts that were a little long, but they prevented the rear quarter panel from getting damaged due to the generous arch of the leaf springs. The 7 1/4 axle can only handle up to a stock 318.