I had pretty much the same thoughts, Michael:
18" tall is not really enough when you are dealing with a 36" deep cage - you won't be able to reach the back without leaning inside. 2ft tall allows good access...3 ft tall allows you to put small children inside to do the cleaning for you
The hide - I haven't kept a snake that size in 10 yrs, so I'm having a hard time combining memory + spacial relations. You see him all the time, so I'll accept that the dimensions will be sufficient. It will, however, detract significantly (too much, IMO) from the floor space of the cage. Michael offered a great idea - namely having the hide below the cage. You could use the other portion underneath for supply storage, thermostat housing, etc.
There are a few different options for heating, though FlexWatt (Calorique) is probably the cheapest approach. 6", 17", and 24" widths are also available. If you are planning on keeping the tape only in the covered portion, will that provide high enough temps for the rest of the enclosure?
Can't wait to see the finished product!!