A 250 in an AD truck is a very doable engine upgrade but there is some cutting, welding and component relocation involved.
I recently replaced a 250 in a 49 GMC and took some pics and made a few observations as I went. I'm no expert on this mod and welcome any corrections.
I never got to drive it with the 250 but have driven 250 powered Chevy's back in the day and a 250 should have no problems moving an AD truck in modern traffic.

As stated the 250 is long and the bell housing taller, especially if it's an automatic so it won't fit without some mods.
If you want to go with an automatic or more modern straight stick you'l need to change the rear and driveshaft to an open type. A popular donor is the 70's-80's Nova's. This will involve cutting & welding on the frame and axle housing. May have to have the driveshaft cut, rewelded and balanced. Due to the need to do it correctly it is a 9 on a scale of 10.

The radiator on a GMC AD is already setback due to their original OEM engines being longer than the Chevy's so the radiator on a Chevy AD will have to be moved forward. There's room in the radiator mount and it is a 3 or 4 on a scale of 10. A possible easy mod may be to change to a GMC AD radiator with it's built-in setback brackets. One would have to research that.
The 250 will sit too low to use a mechanical fan so going with an electric fan is necessary.

Pic of how close the waterpump pulley sits to an OEM GMC radiator: PIC

Another pic showing how tight the engine sits in the bay: (Note the small 12" electric fab has clearance but is too small.A larger 16" fan would fit: PIC

A pic of the right side motor mount: PIC
Left side motor mount: PIC

A pic of the firewall cut and also shows where the frame mounted motor mount brackets were welded. Also take note that the frame cross member that was located basically right at the front of the cab has been cut out to make clearance for the 250 engine and TH350 tranny. The replacement crossmember is further back under the cab:
PIC
Another PIC: PIC

Another shot of the frame mounted motor mounts. These are OEM clam shell lower mounts: PIC

A pic showing the 250 and TH350 when it was pulled. It is a fairly long powerplant setup:
https://photos.app.goo.gl/RkxtZCH1sRKCznmZA

Good luck
Dave



1949 Chevy 1/2-Ton 3100
1949 GMC 1/2-ton 100
In the Stovebolt Gallery
More pix on Photobucket
Video of the 216 running
Video of a Winter Cruise