How much slower would a 29 outlaw with twin sbc 300 hp be versus one with BBC 310 hp be?

I think this is more an issue of weight than HP in a 29. The BB's will weigh at least a couple hundred pounds more than the SB's. In a 29 this will have an effect on hole shot and I wouldn't expect more that 1-2 maybe 3 MPH more top end with BB's.

My 292 has twin SB's (6.2 MPI) 320 HP a side and runs 70 MPH.

I would say you don't see much in the top end at all. maybe 1 mph if any. Hole shot will be better with the 7,4's. Much more low end grunt. The weight diff. should be 240 lbs. in all.

I got 62 in my old 29 OL with 7.4s. 23" Mirage 3 blades.

What kind of cruise speed and rpm at cruise did you get out of her?

Seems like speeds tended to vary with the 7.4 powered 29s. Dad had a '97 model with carbed 7.4s. Came with 25" Mirages and ran upper 60s all day long.

Mine was a '99 efi. Not real sure of cruise speed and rpm. I knew I wasn't going to win any races. Only had it a year. I usually ran it 37-3800. Id guess just under, or at 50.

Thats what I would have guessed. Dad's with the 25" props ran 49 at 3500 rpm. Mine with 26" 4 blades and 496 mags only runs 47 - 48 at 3500 for some reason.

I guess it was the 454 mag with 365 hp then. The 7.4MPI 310 hp doesīnt turn 25īs.

You should try a set of Revolution4 25". More bowlift. Remember your 26" B1's are more like 25's, and run flatter than Rev4's and M+'s. 15,25" dia. is also alot of prop to turn with the mild power of the 496MAG's.