Rebuilding my 440 for my 71 R/T

[71green go]

Need some advise...My car had 69,000 miles on the odometer when I bought it, but it was taken apart as someone had attempted to start a restoration of the car........the Original numbers matching motor was already pulled from the car and was sitting filled to the brim with oil for storage, I was told it ran OK but had lost some power...I planned a complete restoration of the car anyways and planned on rebuilding the motor..
I took the motor apart today and found the timing chain was hanging really loose and the gear had the plastic teeth missing from several areas...no wonder it was running crappy
What I was thinking is just rebuilding this engine completely stock as it is te numbers match to the car and I would hate to blow it up.....so is it feasible to think I can reuse the factory pistons and just have the block cleaned re honed new frost plugs and bearings?......I would get the heads done and rods resized, but don't want to re bore it...it has a small amount of a ring ridge at the top of the cylinders how can I measure the tolerance of piston to wall gap ?.....do I have to have the ring ridge removed to insert a feeler gauge?...and what is the max tolerance.....can someone explain how I should go about checking this?........all the rest of the motor seemed good, very little wear on bearings, although I will have everything redone ..

I plan on buying another 440 motor to build up and play with, I just want this one done factory...sound like a good approach?