My 1977 Nova had a professionally built 383 with 60899 heads, mileage on longer drives was about 18 mpg, about what the original 350 got.
Engine had Comp 280H flat cam, dynoed at 430 Hp(5500rpm) 470 torque-nice engine, easy to start and drive. 60899 are 64cc straight plug RPM. I sold this engine and moved up to a 420 Dart. This car is a summer drive/cruise car, hasn't been to the drag strip. Engine was built and tuned for premium (91 octane) pump gas.
Here is a location to search your 6061 and 6073 heads:
You have 4 digits as opposed to newer Edelbrock numbers of 5 digits, so your 6061 will be 60619 and 6073 will be 60739. You will see that both are 70cc, straight plug but 6073 is for solid flat tappet and hyd roller. You will have to do a bit of searching to find spring rates.
Hope this helps,