The 305 and the 307's are two different engines. The 307 I have heard a lot of bad press on but have no real experience with it. The 305 is a smog engine from the 70's and 80's. It's a low compression, low horsepower engine that was usually run in light duty trucks and cars. Niether one is a high torque engine that you need for your big rig. A big block is probably the easiest upgrade since you are aready running a V-8, but a large cube inline will also install with some easy modifications and pull a bus around pretty well.
You'll hear a lot of different opinions but run the 305 while you find your replacement,
Good luck,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!