You saved the original block, so it's obviously important to you. Build it, because when you sell the car, odds are the next guy is going to lose it somehow. Build it with well ported stock heads (because the casting numbers are right there in front of God and everybody to show the world if you swapped them), stroker crank (445 ish cid), Ford aluminum PI intake (looks correct, lighter) and CJ manifolds ( looks correct, better). Make the Toploader a wide ratio box if it isn't already and go.
If you should happen to blow up the original block-oh well. It's already on it's second life and 68 Mustang 390 GT buyers care about 1/10 as much about original blocks as '68 Camaro buyers do. Between the VIN and the Marti report, there just isn't any doubt in what it is.
My $.02
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