It looks like a number of you have swapped out flatheads for slants in various late 40s and early 50s cars, has anyone done this in an old B1B pickup truck? I'm not quite to that point, but thinking about it.

The truck has a four-speed manual transmission.

It can be done but the original 218 is a more compact motor. You will have problems with the length. Its been done but it isn't the easiest swap. The '48 to '53 1/2 ton Pilothouse trucks have compact dimensions and have a short turning radius due to the 108 inch wheelbase on trucks like your B1B, with its cross steering they turn on a dime, but a slant six will be a tight fit. Don't use the OEM four speed.

I have seen it done, but that was a long time ago and they used an automatic. But hey, my Dad and I put a 241 Hemi with 3 speed/OD in one when I was a kid.