If you can post some more pictures I can ID it for sure as a 51 or 53 series. But from what I've seen so far I'd say it's a 51 series. You can pop the "valve cover" and look for valves. The 51 series has NONE, just some injectors under there. The 53 series will have 2 or 4 valves per cylinder.
It's a conversion one way or another, the 53 series came out in 58.
A conversion can be done very cleanly and appear factory, that's the goal of many who do diesel conversions, it's certainly my goal.
As I said earlier I've seen literature for a 4-51 in a GM truck (I'm sure the real thing is extremely rare if any survived). So you could have those parts from an older truck and or a surplus 4-51 engine destined for a truck and now swapped in to the later GMC still making it a truly bolt in affair.
Badges on the hood are just that, I wouldn't judge if it's factory by hood emblems. And the radiator shutters can easily be swapped from another truck.
In the end if it is still a 4-51 you have an interesting truck but not something I'd want to swap those nice/rare NAPCO and Swivel frame stuff to. The 4-51 while interesting is an oddity and likely to be expensive if it needs the least amount of work/parts.
Here are some 4-53 (with turbo) pictures
https://picasaweb.google.com/118082002072608219229/48Chevy453TConversionProgressPictures02Grigg
