If your GT5S is bone stock, numbers matching then if it was my car I’d keep the original motor for sure. But the original motor by todays standards in a supercar is grossly underpowered.
I’d take out the original engine and keep it on an engine stand for future consideration. This is done in the collector corvette, mustang and mopar communities. If you plan on driving the car regularly and the odd track time then putting in a 351W stroker is a great option. Now the GT5S can keep up with the big dogs. When I did my engine swap I only got $1,500 for the 351 Cleveland that was in it (non numbers matching). I had Jerry at PMI Motorsports do a bunch of upgrades to the tune of almost $50,000. My car still looks very stock outside of the 17” billet Campanola’s. I wanted the 335’s on the back that just completes one of the meanest rear ends of any car ever made, besides the engine the rear half shafts, bearings and universals need to be upgraded to handle 600+ HP. Jerry offers a set guaranteed for life to 700HP. The ZF transaxle is fine as long as the pinion gear is wired. Your car is very rare and pulls much higher sell prices so that will factor in. Personal choice. As long as you keep all the stock parts a future owner can always put the car back to stock, I wouldn’t know why though once you experience the upgraded performance. I’ll attach a couple of pictures of my car. If you look on Bring a Trailer some of the highest prices were for highly modified Pantera’s. On Pantera’s tasteful functional upgrades do not diminish the value. Opposite to Ferrari or almost any other supercar made today any major modifications like an engine swap would surely tank the value and be frowned on. My 2 cents 
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