While its fun to dream of all that power just don't consider the motor. What are and in what shape is the rest of the components of the drive train in? Big torque output (what a stroker does), unrestrained throttle foot and unexpected traction will start popping 40+ year old parts like a pan of Jiffy Pop on a hot burner...
I'm pretty leery of "deals" like this. More often than not they end up costing way more in the end than using what you already own. Without knowing the components of the motor, cam specs, head flow #s, quality of machining ect... ect... it's a crap shoot for any of us to advise you. Do you know and trust this engine shop/machinist to give you the straight poop? Do you know all the ins and outs of properly building a stroker motor and all the questions that need to be asked and the answers verified by someone knowledgeable that you trust with your $? I'd pass.
Don't know what is behind your goal of 400 HP but it shouldn't be too hard, or expensive, to get that 340 to make very respectable and street usable power.
