Shouldn't be a problem to build a 273 if you have the desire, basically gonna take similar parts to do the job, except valve size, your gonna have to keep it around 1.78-1.92 which means your gonna have to work with one of the factory heads, and building decent cr is a bit of problem with smaller engines but besides that 350 hp should be relatively easy with 360/5.9 heads and 400 hp be a bit more work, some porting probably gonna be needed. Other than that should be pretty straight forward.