They just stick early guts into a later case.
Still old 19 spline , done it a few times.
Delenzi converters does it for me, about 180-200 bucks for one cut higher stall.
That way it works with the trans yet fits the larger register of the later crank.
My thought was he get a later yr 318 and build it. No sense building a 273 these days unless you have a real commando 4 brl car or something. No one will know cause it looks the same.