The 300 and the 240 are identical blocks,the heads are different and the 240 is basically a destroked 300 so it will rev a little higher.
The 240 and 300 were truck engines,the 240 found its way into a few full size cars in the sixties,pretty rare tho,the 250 was the car engine.
One of the weak spots on the 300 is the ten inch long pusrods tend to bend under high spring pressures and high rpm.I had crane make me double walled hardened pushrods to remedy this.
The 300 head needs some serious porting to make it breath properly.
No one makes any good valve train parts, but big block chev roller rockers can be made to fit the ford head,guide plates have to be hand made.
The inline six is almost a perfectly balanced engine by design,compared to a v type engine,weight match all parts and balance rotating assembly and they will easily handle 7000 rpm,not bad for a six cylinder with a four inch stroke.
It's hard to find good pistons for them at a reasonable price,but if you bore it .050, 390 pistons will fit,they are easy to come by and cheap,the pin size and spacing is identical,so they are a drop in fit with no machining.
Most 300's came with forged rods and the crank is indestructable.
I could go on and on,but you get the point,I like the sixes.