Just bump the charge weight that is shown in 38 special load data for the bullet you're using . Lets take 148 HBWC loads for instance , classic in 38 special is 2.7grs Bullseye . In a 357 case with a 148 HBWC use 2.9grs Bullseye . To get 800fps with a 148 HBWC in a 357 case use 3.3grs Bullseye with an OAL of 1.360" . Slower powders you'll have to figure out , but just bump powder charge .2 - .3grs to start with . Another classic load for 148 in 38 is 3.1grs W231 , in a 357 bump it to 3.4grs to start & adjust to desired velocity . Don't go crazy as with a HBWC you can blow the skirt off the bullet & if you shoot another behind it you'll bulge your barrel . Personally I'd use a 38 case , bump load .2 to .3 grs & seat bullet out of case neck to an OAL of 1.360". Can do the same with a 158 SWC or RN only use OAL of 1.455"with say 3.7 to 4.0grs W231 or 4.7 to 5.0grs of Accurate # 5 .