4l60e transplants?
Hey man, maybe I can help... i used to work at a late model performance shop that delt mostly with 4l60E rebuilds. Here is your first problem, a 4L60E is an electronic trans that is controlled by an ecm. This ecm trans accompanies a fuel injected sb chevy either being vortec/lt1/ls1. Depending on the year determines which computer is needed. The line pressure and shift points go directly off of tps and speed sensor. So right away if you have a 4L60E your going to run into a problem if your not using a ecm.
Now however there are now kits that replace the electronic modulator to a vacuum modulator. This kit will take the computer out of the shifting (which IMO is the better anyway) I wish i knew the company that made it, but im sure you can find it online.
Now if you have access to a 4L60 (no "E") which is essentially an 700R4 just a newer model. Line pressure is then controlled by a detent cable much like a 727 or 904.
It will be far from a bolt in application, of course you know you need a bellhousing adapter and you will have to have the old bellhousing cut off in order to place the adapter on. You will have to make a custom crossmember as well as a custom driveshaft.
These trans built to the gills will only support about 475-500 hp. We usually got about 10-15 10 or 9 second passes out of our stage 3 4L60E's before a rebuild was required (motors over 500 Hp)
IMO, i would head more towards a 91-92-93 truck 4 speed trans if your looking to have that 4th gear for highway crusin. Much stronger, bolts up directly and can be built to handle much more power.
Without seeing the trans in place im not sure on your tunnel. The 4L60E may be shorter yet wider than a 727, im gonna guess it won't fit in the tunnel without some work, but im thinking it will take some modification