I have had two trucks and one bronco (all 1978 or 79) with the 400. I have also owned one 78 two wheel drive with the 460.
I rebuilt one 400. The performance improved, but nothing to get too excited about. It still wasn't as good as the stock 460 in the pickup.
I also replaced a tired 400 in my 78 bronco with a stock 1970 429 with a 2 barrel carb two years ago. I will never mess with rebuilding a 351/400 ever again. If I am going to pay the money I am putting the big block in. If my next vehicle has the 400 in it I will leave it until it needs rebuilt, but I won't put a dime into rebuilding one.
Of course, that is just my opinion.
1978 Bronco Dana 60 front Dana 70 rear, Jed's Machining doubler, ORD triple stick kit. 38" Toyo's. Next winter project will be lockers!