It's all a matter of supply/parts availability.
If you are building a 4.250 bore side-oiler, you have three choices if you want a new crankshaft:
- 3.984 (428 crank) but it uses Ford style rods and limited bearing availability
- 4.125 with Chevy rods and bearings
- 4.250 with Chevy rods and bearings
No one makes a new 3.784 crank, so you have to go with a used one or pay dearly to have one custom made.
The 428 crank (3.984) uses Ford rods and the same original size bearings. When fitting the crank, you have to buy complete sets of bearings to get the sizes you need. Depending on how things workout, it is conceivable you'd need to buy multiple sets to come up with the combination you want.
With the two longer strokes, they use a chevy sized bearing and are available individually so you can mix & match as needed. Also some may argue the longer rod gives a little better geometry.
I'm in the process of building another FE and would love to go with a standard stroke/bore unit - but it just isn't doable - at least I haven't found a way to do it and still use new parts that my budget will allow.
Paul