what modern dodge frame can you drop a 1948 dodge truck half ton on to?

I don't think any Dodge frame would fit nicely. I strongly suspect they are all too big, even the Dakota.
Look at a Chevy/GMC S-10 frame instead.

Plus 99.9% of the time,the Dodge drivetrian is replaced with a Chevy drivetrain due the prices of the parts so cheap,interchange of parts and parts are so much easier to get for a small block and big block Chevy.

I agree with magic Rat