I recently built a 1/62 Vostok, using BT-55 and BT-60, cardstock, and misc. scratch parts. I have flown it just 4 times, but all flights were good, with a tendency to weathercock into the wind, though. Finished weight is about 10 ounces, thanks to lots of nose weight for stability. It flies on D12-3's, the last flight was clustered with a D12-3 in the core and (2) A10-PT's in the strap-ons (I am going to shoot for a D12-3 + four A10-PT's on the next flight, a 'full-up' cluster). The CG for flight is about the midway point on the core stage's rearmost transition (the transition where the strap-on nose cones attach). All components, including fins, were built to scale, i.e. no oversized fins or anything to augment stability. I am now working on a 1/40 Vostok, which will fly with a cluster of four "C" motors in the core stage, plus optional plugged A10-PT's in the boosters.
If you do a search on "vjp vostok", you'll find the threads, with photos and discussion on the building techniques I used.
Also, follow the URL to my homepage (below) and there are videos of 3 of its flights. All the flight videos were with the single D12-3 motor, altitude reached was only about 250 feet. Chutes got fouled on the first two flights (tx to VERY hot ejection charges) but 3rd and 4th flight ejections were perfect.
The Vostok is an ambitious, complex rocket to model, but the finished result is beautiful both for display and in flight. Do it!
If you have any other questions I'd be happy to share what I know.