So, I have a 1969 F85 with a th350 which is hooked to a 350 engine, and was wondering if it is plausible/ how much it would cost to put a 4 speed manual in it.

 

Very plausible .... but cost ??? Which tranny do you want to put in, and how much of the work can you do yourself?

 

Im switching over a turbo 350 tranny over to a 4spd in a 70 cutlass. Costing me about $4000. That's with a new 4 speed M22 rockcrusher Autogear, Centerforce clutch and pressure plate, drilling crankshaft for pilot bearing and other parts to convert over. Not including the price of a SFI explosion proof bellhousing.

 

I got a 4 speed short block for sale with lots of goodies for a very good price. .030 forged probe pistons. Its in the forsale section for parts.

 

I'd suggest you start by using the SEARCH function, as this has been covered about a bazillion times here. Bottom line is the major problem is that the crank in your engine won't have been drilled for the manual trans pilot bushing.