What are the differences between front and rear clutch frictions? They look remarkably similar to me. The only difference i noticed was one set had a tooth missing.

Fronts are .086" thick, rears .061". Rears are generally smooth, fronts are grooved or a checkered pattern.
Doug

I'm thinking there is a thickness difference & I saved an article in the Jan 1999 "high performance mopar" magazine regarding using thinner frictions to use 6 clutches in a 5 clutch 3rd speed (their words) drum from a 75-79 360 eng. 10 second drag raced 904 based '66 valiant. I have one to do & I am going to Google it.

I use 5 thin ones from the rear in the front to get 5 disc instead of 4...

I also have 5 in the front and 5 in the rear. I have access to alto reds @ 0.61 grooved but can they be used in both the front and rear drums? This is a street strip deal 450 hp.

OEM the rears were more aggressive and smooth since it's not a shift element. Fronts are waffle or grooved for the fluid to escape on apply. Now you can get about any configuration. A good performance material or the blues. I use rears (.063) in both to get 5 in the front. You may have to juggle steels and maybe trim the pressure plate to dial in the clearance.

The blues are 2 x the price of the Alto reds. Any benefit in using the blues over the alto's?

what clearance(s) should a person shoot for (front/rear)?

Not that I've heard, at your power level a standard style friction material will hold up. Getting the clearance right is the important part I was always told.

I use 5 .061" smooth red Altos/Kolenes in the high clutch, .012"-.015" clearance per friction (.060"-.075" total). The rear uses 4 factory style silver frictions/factory steels at .025" total clearance. This package currently has 350 passes, 9.1@150, 3350lbs.
Doug

Raises a good question. How much power can standard frictions take in a 904?

The 1st Pro trans I ever serviced had the standard silver rear frictions run in the front location. 727 with 904 guts.
Doug

Doug thank you (& noted!)

How long ago was that? That was an old trick before the blues, Altos, etc. were available, since the rear frictions were typically more aggressive. Any serious unit I've seen, (Stock/Super Stock, low line pressure, high horsepower) has had, or I used, blues, front and rear, along with Kolene steels. I keep the rear clutch clearance snug, maybe .025. Front, depending on trans brake or not, 010 to 012 per friction.

Long time ago for sure. But I opened up for inspection at 100 passes. The clutches looked like new. Put them right back in. Car made 800+@3400#.
Doug

Is that .010-.012 with or without a trans brake?