Jim,
You're right, of course, most HS6 needles won't work in HIF6s, but later production HS6s used the spring-loaded needles as well. That may only have been on 140s with the B18s and not on 122s as original equipment. With all the carb swapping that has gone on over the years, however, you never know what may show up on a particular car. At any rate, as far as I know, spring loaded needles with the right taper for a B18 with a C cam should be available. But getting a set of them may not be worth the trouble and the cost, particularly, since the problem may lie elsewhere. So, yes, going back to the HS6s is probably the most sensible thing for Jack to do: if only to determine if the problem is in the carbs.
Bob S.