700R4, Is it necessary to lockup the Torq Conv?

['68OrangeSunshine]

I used to run a '67 K/10 Suburban w/ 454/TH350/NP205/12-Bolt 3.73 posi. PO's rustic-engineering. It ran good on the straight and level, but in the hills, when I tried to downshift and use engine-braking, the TH350, would automatically upshift. Since it had drum brakes all around [and '67 4x4 front drums are rare parts], stopping was more a situation of selecting an alternative soft target.
Would Lock Up give me positive downshift engine-braking capability? I've thought of ressurecting this beast with a new engine and a 700-R4.