You should not give advice for 273 engines. There is so much wrong information in your post. The 273 was the original LA engine. They were never a smog motor. The 2 barrel pistons were almost at zero deck and were almost 9:1 compression ratio. Closed chamber heads were used from 64 to 67 with solid lifter cams and valvetrain. The 64-65 heads have a different intake bolt size and angle than all the rest of 66 up LA engines. Solid cam and lifters easily last 300,000 miles if you use good quality oil and filters and change them regularly. A 318 has a larger bore than a 273, but to keep compression low they dropped the piston in the bore and used open chamber heads in the 273 and 318 for 1968. To keep the compression ratio in the 273 the same, they raised the piston in the bore and that brings the 273 to zero deck in 1968 and 1969. As for the cams, I'm not talking theoretical numbers. I have run them, and helped others who were happy with their 273s.