I shoot a 454 but it’s not a Super BlackHawk. You will get the same buildup firing a 38 Special round in a 357 Magnum as you do with the 45Colt in the 454. The shorter round will leave a burn/carbon ring in front of the throat and you may have difficulty in loading the longer cartridge. It will raise the pressure a bit but I’ve never heard of any problems in revolvers with the exception of the Freedom Arms 454.
As for the primer, use whatever your reloading manual calls for – I’ve always seen SR magnums recommended.
Another option, through a little more expensive, is use 460 SW cases trimmed down to 454 case length and use the WLP primer.