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You can not use the 318 roller lifters, there is nothing in the block to hold them from twisting and destroying the cam.

You can not use a 2.02 valve, the notch required to get clearance much less unshrouding the valve will hit water.

You are not the first to try this, lots have tried it and an overwhelming number of them say it is very disapointing when done. The displacement is too small to make much TQ and the bore size is too small to make much HP.

The stock 273 heads are the best flowing stock head that will fit a 273 bore, use what you have if your budget is that tight. A little mild port work is fine, you can add 1.88 valves and a small notch at the top of the bore for the most flow. The 302 head flows less and will give you less compression and more crevice volume, all of those will result in less power. Mill the block to zero deck the pistons, run the .039 fel-pro head gasket. Get a comp 270S cam and run the factory adjustable valve train. This will be about as peppy as it will get without a bucks down build.

BTW
318 2bbl 230 HP 340 TQ
273 4bbl 235 hp 280 TQ
360 2bbl 255 hp 360 TQ.

Not trying to tell you not to build the 273 just letting you know what handicap you are working with.



I think most people who are going to want to build a 273 are the ones who want their engine to match their car (like on an early-A) or those who want to be different (someone like me ).