Tim: Are you sure the end ''bolts'' are broken? OEM L6s use non-threaded studs as guide pins on the ends of the stock manifolds. These pins keep the manifolds in place and allow for expansion and contraction during operation [hot] and shut down [cooling].
The head manifold mount bolt holes at #1 and #8 are threaded 3/8-16 unc. Since I run aftermarket Clifford headers on my 292, I have replaced the guide pins with real bolts, since the expansion/contraction idea doesn't work for headers.
On a stock manifold set up, guide pins are perfectly normal.
For one-barrel air cleaner ideas, you might ask at
www.InlinersInternational.org
or
www.12Bolt.com
or
www.cliffordperformance.com
I've been running 4 Bbls for 30 years.