I removed the insulation covering most of the firewall (fender to fender); it was falling apart as is prolly common w/ many of our trucks. I then covered this area with "peel-n-seal". The smaller/separate insulation just around the bellhousing area I covered in Al tape (like used to seal hvac ducting) to hold it together enough to be re-used.
Peel-n-seal is intended as a roofing patch material & comes in a roll & is a thin asphalt layer with Al foil backing & the asphalt side has an adhesive. This product has been used by car audio enthusiasts as "poor man's dynamat".
I can't provide any head to head sound damping comparison to dynamat/hushmat, etc.,. But can say it quite noticeably quieted my truck's interior when I put down 2 layers to the cab floor before new carpet. I put it on the firewall because it was easy w/ the engine out & wouldn't absorb oil/fuel etc. Do think it would look better if after applying the peel-n-seal, I'd sprayed the Al foil outer layer w/ a black product like Lizard Skin.
I suspect the open, fibrous nature of the factory firewall & hood insulation is likely to be more effective at keeping engine noise out of the cab. But replacing the factory firewall insulation means work beyond just pulling the engine as I'd imagine Bobbie can attest to.