Just thinking out loud, here.....one of the tough parts of the 27T body build is getting the driver down as deep as possible, keeping in mind that the foot-floor cannot be below the bottom of the body line. That driver area is very narrow after you get cage tubes inside the body, and the driveshaft is now extremely close to the drivers seat. Building for rear suspension travel means either compromising the drivers position (and cage height sticking in the air) or raising the car high enough to allow that driveshaft to have up and down movement without hitting the driver. In addition, you need enough room to shield the drivers hip and kidneys from an exploded U-joint.
We were warned about that by many successful builders and so decided not to amplify the problem by letting the rear u-joint move up and down. Mod Roadsters can have the drivers back all the way to the rear axle transverse center-line, which puts the nose of the diff right where you would really like to have that space for the drivers anatomy.
Keep in mind that the engine/trans will be sitting higher than you expected, which means the front end of the driveshaft is pretty high compared to the drivers bottom position. We actually used a Toyota diff with "offset to the right pinion", and mounted the engine/trans to the right to gain what we could. We also pushed the engine forward to gain a little more room in the cockpit for that intercooler tank. Doing that step also simplified the firewall and left more available firewall area to make good, fire protected pass-throughs for all those water lines. 27T doesn't have much firewall area to play with.
The solid rear axle mounting made it easy to set up the rear brake components and have enough looped flexline length to be able to pull the wheel/axle/backing plate packages far enough to swap out differentials without complete teardown or opening the brake fluid lines. It also left good room for the parachute pull-point mounting.
Jim Deist instructed us to put the pull-point height (at the back of the body) on the line of a string pulled from the vehicle true CG, 35 feet back with the end of the string 7 feet above the ground. That worked perfect, over the years. If you don't build to go fast enough to use a chute, you will be missing out on some of the fun! It is soooo cool to feel that chute grab your car....you'll like it every time.
I don't think the traction advantage of rear suspension will overcome the added aero drag of putting the driver and cage too high out of the body. Those round tube roll cages have ferocious aero drag, and this becomes a case of "less = more" by minimizing that exposed height. You can't do much about those open wheels hanging in the air, so the only aero you can improve on inexpensively is total frontal area. We all pretty much use the same body....it's the cage and windscreen choices that have the biggest effect when you start going fast. Get that driver as deep as you can, within the rules....and then you build from there.
Re: tanks....we ran a water to water intercooler tank just to the right of the drivers legs, and a 16 gallon cooling tank in the right side of the cockpit cage area. We used heavy sheet metal shield around that big tank, which was great that day at '99 Speedweek when the boost ran away, blew the head gasket and torched away parts of the head and block, and exploded the water connections into the cockpit tank.
The true cause of that mess was the necessity to solid mount the engine...which vibrated apart some linkage. It's always some dumb little thing that makes the biggest problems.
Over my years on the salt, it seems to me that if the salt is so rough that you need suspension....it will probably not let you get up to a well established record. The better years make a big difference, because a lot of the good records were set on decent salt. That just seems to be the way it goes, and it doesn't go as well..... as time has gone by. My first Speedweek was wonderful salt, all week, back in 1969.
.....sorry folks....I ramble on as the memories come back. It's called old-age and "decrepitude", as my sister used to say. Thanks for letting me ramble.