A W72 400 would be a great choice for a 79 TA, but they are in demand here in the USA. Finding a rebuildable core is possible, but to have one rebuilt $4000 including the core is going to be difficult, since the total cost- core, parts, machine work, assembly-could easily exceed your budget.
Stock W72 400s were made from 77-79 and had a factory output of 220 hp. You want to avoid the early 77 W72, which used the weaker no.500557 400 block. PMD had problems with this block failing in regular use, which is why they switched to the earlier no.481988 400 casting in 78- much stronger and more reliable.
I'm not aware of the emissions requirements in France, but if you don't have to pass emissions testing a W72 400 can be built to.make 350-400 hp using the stock 6x-4 heads and 3.75"stroke crank. Of course this requires high performance parts like forged pistons, rods, high lift/duration cam,higher flowing manifolds, as well as some head milling to reduce the combustion chamber size and increase compression.
Geno