While you’re at it, don’t reuse the old GM-style starters, get a modern gear reduction unit. Your battery and engine will thank you.
In addition, if you fit the Impala with a TH700R4, your acceleration will benefit AND you’ll get up to 40% better fuel economy.
Those trannies got a bad rep because they got used in the Blazer and burned out alot.
The engine in the Blazer is way, way too small, so the tran does alot of shifting - UNLESS (and I learned this the hard way) you have the throttle cable set right.
(needs to be set so the tran won’t shift up unless the engine r’s are higher)