Seabee: I load many of the same bullets for both 357 & 38 revolvers & my general rule of thumb is 10% more powder in the 357 case to achieve close to same velocity. That's really not a whole lot when you consider say that a classic 38 load for a 148 HBWC is 2.7 grains of Bullseye & 10% more brings it to just 3.0 grains. All 38 Special standard pressure loads are way milder than modern 357's can handle & there's no way your going to hurt a Ruger or an S&W or Colt 357 Magnum with a load with 10% more powder than the 38 load. SAAMI max pressure spec for the 357 is 35,000 PSI, more than double the 17,000 PSI spec for 38 Special standard loads, 10% more powder than the 38 standard pressure ain't gonna hurt nothin'.
For OAL, stay with the crimp groove and a roll crimp if the bullet has one, lead bullets usually do, some jacketed & some but fewer plated do also, but if NOT, max OAL for the .357 Remington Magnum cartridge is 1.590 in., although Smith L frame cylinders can take rounds that are longer than that, but Colt's can't. But the idea with the 357 case versus 38 with light loads is to reduce internal free space for better ignition consistency, so accuracy may improve with light loads by seating slightly deeper with non-grooved bullets, maybe to just before the start of the ogive & a slight taper crimp so you don't cut into the jacket or plating.
