Rebuild or swap? 1970 Valiant

keep in mind swapping from a /6 to V8 you'll need to consider all the peripherals: exhaust, motor mounts, transmission, fuel line, wiring, cooling just to name a few.

then you'll need to track down all kinds of fiddly bits like kick down linkage, throttle cable bracket and return spring mount, a new throttle cable, things of that nature.

even if you sourced a complete take out air cleaner to oil pan with a transmission attached you'd still be looking at several hundred dollars in associated costs.

if I were in your position I'd break down the one motor and have a machine shop do the head and rebuild the bottom end and supply it as just a short block. that way you can do most of the heavy lifting and learning along the way and not be too outside of your comfort zone.

that'll get your wheels back for not a ton of dough and you can decide if you want to dig deeper and do the other motor or save up and collect parts for a V8 swap.