With the 6x-4 heads you can have them milled ~.030 to increase compression some, that will help selecting a new cam that IMO will add more power than just using 1.65 rockers and you can keep the stock 1.5 rockers or upgrade to roller rockers or roller tip type but the stock stamped 1.5 rockers would be fine. Stock rocker arms can be made adjustable by using SBC crimped lock nuts or poly locks & adjust like an SBC, 7/16 studs are best but for a lower lift cam the stock studs are usually fine.
You will get a lot more power/performance from a good mild cam than just using 1.65 rocker arms. All depends on what you want from the car and how much work you want to do.
You already have headers and good exhaust system that will take advantage of a good cam. I have a 78 400 W72, but since you already have the 6x-4 heads your motor is the same compression as a w72 just not the same cam... and the block is weaker than the W72 XX block, but for what youre doing that is not a concern, 500557 blocks are fine for street use under 400ish HP.
Some members on here dont like the comp XE cams but they are designed for lower comp engines and if tuned right they work very well in these lower compression engines, i have the xe268 which is a mild cam with ~.480 lift, can use common valve springs from crane or comp, i installed it "straight up" and so far only real tuning ive done is a Q-jet carb built by cliff, but stock timing curve, stock intake with headers and 2.5 exhaust... car runs excellent in all street conditions and runs mid 13's at 103+ mph with old hard BFG street tires & taking it easy on a new at the time clutch, plus its a lower mile #'s matching car so wasnt going for max times, just a fun test & tune day at the dragstrip.
Im sure there are better cams and maybe some others will suggest some, but for a very basic mild cam change, the car runs very good & does respectable times for a low compression 400.
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