5.9 Mag stock LB or rebuilt SB

I need you alls opinions. I’m looking for a 5.9 magnum for my 69 Dart build. I’ve found a complete stock LB with supposedly 96k miles on it. I’ve also found a rebuilt .020 over short block with new stock cast pistons, new lifters, cam (not one that I would use), oil pump, etc. fully assembled and never installed. Prices are close enough that it’s not a decision factor and I’m going to replace the heads regardless so the long block vs short block isn’t a big deal either.
My question is which would you prefer to start with? I’d like to get to 400 hp and this will be a street only car, no racing. Do I need to replace the stock pistons to get the CR higher to get to that HP or can I get there with stock pistons, aftermarket heads, intake, cam etc? I’m leaning towards the short block, but I don’t know the quality of the build. If I go with the long block I was hoping to get by with just new bearings and a hone.
I appreciate any feedback you all have.