B engine in early Valiant

As bOb said, the 360 would be the most convenient swap, and actually goes in there quite easily, especially since you already have the 273 in there. I swapped a 360 in to replace the 273 in my 66 and it was a really easy fit. You can easily make more power with the 360 that you can successfully put to the ground in the early A's, unless you minitub and relocate the springs.