Setup for 273

There is nothing wrong with the stock 273 adjustable rockers and they should bolt onto your 318 heads.

You are correct about the additional chamber volume lowering your compression. I recommend you find a way to CC the chambers. Every 273 head I ever cc'ed had significantly more chamber volume than spec. 318's may be the same.

Is there any problem with valve interference with the cylinder wall when using 318 heads on a smaller bore 273?