1963 Plymouth Valiant Signet Convertible Restoration...HELLLP!!!

Yes on the 318.

Small block early A headers are EXPENSIVE!! Like $1K a set.

Your 904 wont fit a small block.

The 727 will require floorplan mods to fit in an early A.

Cannot keep push buttons with that trans either.

Will need a different driveshaft.

Can keep the 7.25 rear end.
At least until it explodes....

Easiest path would be the 318 with a cable shift 904 (to keep the pushbuttons) and then you would not need a driveshaft.
And look for a set of early A 273 exhaust manifolds and motor mounts.
That's the cheapest route.