The stock 273 cam is super mild, even the 4-barrel cam only had 248 degrees duration. If it's an otherwise stock 273, the factory 340 cam is plenty big and will be a big improvement in performance.

You friend can either swap in 318/340/360 pushrods and non-adjustable rocker arms, or keep his adjustable rockers and just buy some pushrods ready-made for a hydraulic cam and adjustable rockers (which would be my choice).

Other than the pushrod issue, putting a hydraulic cam in an early 273 is no more or less complicated than changing cams in anything else.