I haven't used slidewize before so can't comment, I have emailed them a couple of times and didn't find them very helpful though.
I have used import monster ( www.importmonster.com.au ) to get a front bar from Japan for the R30 and I rate them very highly.
Plus for the record the RB series is very easy to put in the R30 chassis, everything bolts into where it should so there isn't much engineering to do, if you wanna go putting toyota or mitsu engines in them go for it, but at the end of the day an RB will give you less headaches and you will get the job done quicker.