What is the cheapest way that will work to get a 915 trans working in a 914.

A 915/916 is expensive no matter what.

define " cheapest"

no matter how you go about it you'll have to flip the ring and pinion and spend a bit on shift modifications ( $$$ )

what exactly are your engine plans that you think you need 911 SC brakes and a 915 trans?

A competitive track car isn't cheap. The 914 may be a cheap car but by the time you build everything fast it cost just as much as building up any other race car. 50k can be spent quickly.

Like some thing to chew on:
Conversion kit: Martins = $ 3100 to $3300.
Wevo= $2000 +.
Machine work to clear / flip ring gear....open discussion
Shift rods in car ,....open discussion
915 trans to be rebuilt ( new sliders, dogs, synchro ) reset pinion, $$$$$
Oh how about a limited slip since your there...Guard, $25/2600.00
And maybe short gears too.
Once done, you will have a bang up trans.
With the above, be ready to drop 8-10k.

Good luck

Right on the money. I did one about three years ago. Gear sets are $600 each if you want different ratios.

This is why xperienced people always say to read the rule book first. Or "rtfrb"

The first step to building a race car is to pick a competitive class and weigh the cost benefits. That's how roger Penske does It and how any builder should do it. Find a class and run group before buying parts. Will a 2.7 be accepted in vintage? Generally no. Will a 2.7 be competitive against V8's in an open class? Probably not. Thus knowing your class and competition is the first step.

Then take into account which hardware is necessary for that class and how much does it weigh. Followed by how much it will cost.

My 2.7 is supposed to be around 250hp. I'm just thinking outloud bro. I'm going to run the 901 since my buddy has a bunch as well as knowledge and a bunch of parts. He used to race formula v with them and ran 5 baja races. He is confident that they will work and if they brake we will see where they brake and go from there. The vented rotors because I think I am going to need them. I am going to be using it for a pretty much track only setup. Plus I just like to learn. I really didn't start researching all of this until the last couple months. I like to learn and this is by far the best place for me to be learning info. I have a couple other cars I could have built a track car out of and I chose a 914 because they as well as porsche intrigue me. Maybe for the main reason porsches foundation for all their vehicles was road racing.

I got you. I will start with that and go from there because that engine is what I have to work with for now.