I think tink.kilkee has already tried this cross BUT as I pointed out to them it is wheaten from the Marans and will take a while to get back to Wellies as they should be . I believe the Wellies eggs ( terracota as it is , is also a diffferent colour to that of the Marans eggs . )
In MY opinion the very worst thing you could do is add Brown Leghorn . The ONLY "QUICK FIX" is to buy in birds that have the quality you want .( hence your request here
) If these are not available then it is back to the long haul and selectively breed . Trouble is with Wellies, and Barnies for that matter, there are so many things you need to be mindful of . The only answer is breed 100's and cull hard .