Question is.. will everything still run fine?

It looks as if this truck originally came with an O2 sensor in the manifold as well... ? I'm planning on running the OE 20r Exhaust manifold till I get some more $$$ For a header.

Any help/opinions/experience would be much appreciated. Thank you.

first question, where are you??  this impacts alot.
second, unless you dont have anything else dont run a 20R exhaust, run a 22R stock manifold.  it flows better and will bolt right up.
 propane will run great with a high compression ratio. but you want about a 325 cfm intake so what are you using for a "carb" with the propane setup??