I've done the 350 thing to my '64 and used standard car headers. I'm not sure of the application but the parts store had them on the shelf, so I tried them. The only problem I had was with the clutch linkage.If a '61 is close to a '64 you shouldn't have any problems with headers. There's more room than there appears.
Good luck,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!