Mike - Here's the valve part numbers for your '61 235:
GM Parts Group 0.296 - Valve, intake, GM# 3815082
GM Parts Group 0.297 - Valve, exhaust, GM# 3731212 / replaced by GM# 3839387 07/66 (this is also the standard Chevy small block exhaust valve for that time period)
If you're on the hunt for some, the following are currently on evilbay:
6 each NOS GM# 3815082 intake valves Note: These have some minor surface corrosion/rust which should be able to get cleaned up to put these to work and it's a decent asking price IMHO.
12 each NOS GM# 3731212 exhaust valves Note: These look really good and enough for 2 engines, making the asking price really decent IMHO.
If you'd rather have brand spanking new replacements:
The Filling Station AF-18 List price $12.95 + shipping each.
The Filling Station AF-16 List price $7.75 each + shipping
Happy Hunting!
Edit Note: These are not "stainless steel" valves, just the stock or stock style replacements. If anybody has part numbers for correct stainless steel valves, it would likely be Jerry "Hotrod Lincoln". I'm sure he'll chime in when he sees this. 
