You can use a Y iron in a R5 engine...
 

As long as its a middle iron

what you CANNOT mix, its front and rear irons

New apex seal vs. used apex seal. The triangular piece is new on both
http://img355.imageshack.us/img355/6679/dsc03027cv7.jpg


YES, you can use a middle Y irong in a R5 engine.
http://img355.imageshack.us/img355/6...0081055iy8.jpg

http://img355.imageshack.us/img355/9...0081064om5.jpg

http://img339.imageshack.us/img339/2...0081065yz2.jpg

Porrrrrrrrrrrting

http://img149.imageshack.us/img149/3...0081066rn1.jpg

http://img339.imageshack.us/img339/9...0081071wk0.jpg

http://img367.imageshack.us/img367/6...0081072ho7.jpg

http://img367.imageshack.us/img367/5...0081076gj5.jpg

http://img212.imageshack.us/img212/1...0081077of5.jpg

http://img529.imageshack.us/img529/8...0081080bd6.jpg

http://img529.imageshack.us/img529/6...0081082ew3.jpg

http://img212.imageshack.us/img212/4...0081084pb1.jpg

not exactly familiar with "R5". What is that referring to exactly

R5 is the designation for the 79/80 12A irons.

Y is the designation for the 81/85 irons.

R5 engines use 18 tension bolts

Y engines only use 17

There are small changes depending on what market the engine was going to.

I have various R5 rear plates with 19,18 and 17 tension bolts. It depends on the year they were made. Most nitrided ones lack an oil pressure hole so it takes a steady hand and lots of patience to drill and cut threads.

am i looking at it wrong, or is the tension bolt going to be exposed in the water jacket?

A Y intermediate can't be used with all 19 tension bolts because the casting apparently won't allow it.

I only have 18 tension bolts on my R5...

see

http://img339.imageshack.us/img339/9...0081071wk0.jpg

and I placed a black X where the tension bolt that the Y iron does not support should be.

On the R5 middle iron, (blue mark) you have a tension bolt hole.
On the Y iron (red mark) you dont, its all water.

On the rear R5 iron, you DONT have the tension bolt hole

So I guess it doesnt matter.