most 20Bs are $3-4K thats about the same price as a decent SB crate motor (around 350hp). with both motors you will need to fabricate a subfame.
you would need to do custom plumbing with both motors. with the small block you would need you would need to fabricate headersthen weld up an exhaust system, the 20b already comes with the turbo manifold and down pipe, you would just need a mid pipe welded up.
with the SB you would still need a transmission and transmission mount, and custom drive shaft, or just make a custom bellhousing and use the mazda trany and custom fly wheel. with the 20B I believe you can bolt it up to the stock rx7 trany.
you would also need an ignition system for the SB.
you would also need a manifold and carb for the SB, depending on how much power you are making this could cost $500-1500 easy. or you can go with an EFI set up, kits for SBCs run 2k i think.
if you are doing the 20swap from a clip then you will have the harness and ECU, if not then you would need a haltech that will run you $1500.
I just do not see how a SB swap will be any cheaper then a 20B
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