Decent power can be attained from the SBC 400. I had the 400 in my 76 rebuilt back in the Spring of 2011. Motor was completely torn down and rebuilt with new pistons, hydraulic roller cam,headers, Edelbrock aluminum intake, new carb and heads machined for better flow. The motor dyno-ed at 360 hp and 460 torque before going back in the car, which was a substantial upgrade from the original 175 hp.The motor powered the Avanti to both coasts and many other road trips to SDC and Avanti Meets having a few hiccups along the way like not running worth a darn at altitude in Colorado and losing overdrive in the 200R4 trans in AZ during a Route 66 tour in 2015.
Fast forward to this past August, I decided to go with an MSD TBI Electronic Fuel Injection to cure altitude sickness. The mechanic doing the install didn't like the way the motor was running before starting to work on it. I had thought it had been running more "cold blooded" than usual and also was noticing the motor needing about a pint of antifreeze monthly. Tearing down the motor revealed a significant cracked head on #6 cylinder and a small crack on #2 with just under 36000 miles and 6 1/2 years since the rebuild. The motor again was totally torn down, new crank bearings, crank rebalance, new rods and pistons, new roller cam with more EFI friendly lobe separation, new Dart aluminum heads were added in along with another .0010 over, making it a 408 was completed mid September. The motor again was dyno-ed and this time is making 471.5 hp at 5400 rpms and 522 torque at 4400 rpms.
I only had put around 600 miles since the rebuild and EFI switch so final EFI "learning" will need to be completed next Spring after the car comes back out of storage. The car starts very nicely, turn the key and listen for the electric fuel pump to stop pulsing, then start. I had good fuel economy before the 2nd rebuild, getting up to 26+ a couple of years ago on a trip with the 200R4 AOD, but haven't seen those kind of numbers on the few tankfuls I have driven since the rebuild. I'm sure some tinkering will be in order next Spring before everything is running the best it can.
I toyed with the idea of replacing the engine with an LS the first time it was rebuilt, but opted to keep it original. FWIW, it could have been for what the engine teardown, rebuild, and trans switch cost. This time I wasn't expecting the whole teardown and engine rebuild together with doing the EFI, but I stayed with it. Now the "original" 400 is comprised of the block, crankshaft and oil pan, everything else is new.
I'm looking forward to a trouble free driving season in 2018 and am currently contemplating driving the car from Chicago area to Spokane for the SDC/AOAI Meet.