Engine Swap: 225 /6 to small block V8
This is from my experience doing a 64 D100 225 /6 W/3spd Transmission
Note: Buy a complete engine, not a long block, this is from experience.
First there is a couple of things you'll need to make the swap
1. From Richard Jessop;
    Dodge trucks from 61/68 have rear mounts on the bell housing and hydraulic clutches.
    Dodge trucks from 69/71 have mounts on rear of trans and manual clutches.
2. Make sure the crank is drilled properly for the pilot bushing
    The dimension is .9375 (15/16)
    Pilot Bushing dimensions are: OD= .9375 (15/16), ID=.750 (3/4), length .875 (7/8)
3. V8 Flywheel and clutch, this should be 11 inch Beck and Borg, 10 spline
   The 318 Poly and 318 LA engine use the same flywheel.
   The 226 /6 flywheel is a dual patteren for a 10 1/2 or 11 inch clutch.
4. V8 Brakets, these bolt to the engine
5. Front sump pan and pick up tube
    I have seen the stock pan used, there is about 2 1/5 inches between the pan and axle
6. V8 radiator or modify the 6 cyl radiator.
7. The motor mounts need to be moved down to the alternate holes in the frame.
8. The bellhousing from the /6 will bolt to the small block

This is my experiance:
Remember this is my first experiance with these old trucks and with Mopar for that matter. I bought the 64 with a bad 225 /6, I decided if I was going to switch engines I would rather have a V8. A 318 sounded good. I saw an add for one and after talking to the guy I told him I'd take if he he could deliver it. He told me they used it as a smog engine. They would put it in their hot rod to smog it and then pull it after it was smogged I soon learned it was a 273, not a 318. First lesson, don't believe the seller. I learned that the block casting has the cubic inches. I spent way to much time money finding and buying the parts I needed to complete the engine. Exhaust, pully's, distributor, water neck, coil, thermastate and water neck, alternator bracket, dip stick and a few other parts. I thought I was ready, I bought a pilot bushing, it wouldn't fit. It turns out the crank was not drilled the correct diameter for the pilot bushing. My nieghbor, a machinist offered to turn down the bushing. I was offered a 76, 318 camper van with a 3 spd trans for free from a friend of mine. t had 58,000 original miles. I decided that would be the best way to go. I scraped the idea of the 273. The van would give me everything I needed, I already had the flywheel and clutch. The vac has a 3 spd, so it is drilled for a pilot bushing. I'll add more when I'm done and add some pics too.