Fair asking price for a 904?

That combo wouldn't bring enough to buy the floor hump to accommodate the 4-speed.
Start at the rear of the car. Start preparing for the big motor and transmission and build a strong rearend ? With 410's 391's or the very nutral 355's. Might even lifft the spirits of that /6 LOL. And continuing forward, a shorter driveline will be needed, but the 904 and your potential 4-speed are the same length so your all good. Be aware they have different slip yokes but those can be switched on a vice at home.
Ask anyone, that 7 1/4 rearend ain't gonna last 3 minutes behind any v8 4-speed. Continuing forward, I noticed your near Eugene? There was a guy on Craigslist in Wilsonville giving away a 904. And I'm assuming if you're trying to get rid of that slant 6 and work with a budget and a budget here is of high concerns? And going on that theory I would stick with the automatic til you get a full V8 automatic killer and combination together and then start saving for the 4-speed stuff because that's going to be a Backbreaker. You can pick up like everybody saying a good rebuildable 904 4 $50. A rebuild kits $104 a nice shift kit is about $92. The decent torque converter that's going to cost a little but don't discount the idea of taking a stock one and just having it reconditioned. I believe that's very inexpensive. ($75? Don't quote me) For the money it would cost to get the 4-speed pedals and a 4-speed bellhousing you could have a great snappy 904. Moving forward, I assume that you're looking to do a V8 of some sort?
I've been doing a lot of assuming here LOL let's just hope I helped and didn't make an *** out of myself.