gearbox conversion

i think my gearbox packed up and that is not going to be getting fixed. not worth the cost!
one option may be a manual coversion since most of the car is good. new brake pads and lines, tyres, ball joints and exhaust. i still have my old V6 with a lot of bits missing but the gearbox is intact although i suspect the input shaft bearing needed replacing. it made a RRRR noise all the time i had the car but it worked fine!
is this a reasonable option to fix the auto V6 using the manual V6 or is it going to be too expensive to consider? we are talking about a 2000 galant V6 auto which i paid £250 for but i do have almost all the bits there to do a conversion so it really would be down to labour costs plus any seals etc that it would be advisable to replace. clutch too of course!