You did not say what the tank is constructed out of or what the base will be.If this is a wood tank and on a strong stable base then no you are fine.
If it is glass I would suggest a solid sub base for the glass bottom.
If you are looking to replace the entire top try
www.glasscages.com/ or most local fish stores can order.
You can also just bend or have bent a piece of aluminum or stainless to fit across the center of the tank.
Mrchub
I think you might want to detail your tank size, filtration being used, types of fish and your current water parameters.For some species and breeding purposes it is common to change water parameters.
Most fish will adapt to water conditions.
Not dumb question,sure you can do it but why.If you are trying to change water parameters a RO unit will be more effective and less costly in the long run.Most human units use carbon and are costly if using lots of water.
Is the tank 8' left to right with 2 pcs 4' wide Plex for the front viewing area? Is there a 2" or 3"center strip of 5/8 plywood ? You would want a brace at that point as the ply and plex joints would be the weak point. Pic would help.
Using the suggested 5 watts per gallon this would equate to 900 watts. Being in socal where the weather is generally warmer I would think 2 heaters 300 to 350 watts would help maintain the temp you desire.
If you use 2"x4" doubled up and with the 4" facing toward the top then you are good to go.I use single 2"x4"s on my 55 gallon tanks with the 4" vertically and never had a problem.
HTH
If they turn out to be a pair and you want them to successfully spawn and raise there young then a tank of their own would be in order.Your present tank occupants will probably produce a holy war.
Ps Oscars with eggs or young can and will draw blood if you decide to venture your hand into...
Fwiw
I have build at least 5 tank's 8'x2'x2' out of 3/4" plywood using 2" and 2 1/2" # 6 wood screws. The only bracing was 2 pieces of 5/4" x 5 1/2" across the front and back set 1/4" below the top.During assembly the screws holes were predrilled and then filled with a good grade of wood...